Williams Textbook of Endocrinology, 12th Edition

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Shlomo Melmed, MD Professor of Medicine Senior Vice President and Dean of the Faculty Cedars-Sinai Medical Center Los Angeles, California

Kenneth S. Polonsky, MD Richard T. Crane Distinguished Service Professor Dean of the Division of the Biological Sciences and the Pritzker School of Medicine Executive Vice President of Medical Affairs The University of Chicago Chicago, Illinois

P. Reed Larsen, MD, FACP, FRCP Professor of Medicine Harvard Medical School Senior Physician and Chief Thyroid Section Division of Endocrinology, Diabetes, and Metabolism Brigham and Women’s Hospital Boston, Massachusetts

Henry M. Kronenberg, MD Professor of Medicine Harvard Medical School Chief, Endocrine Unit Massachusetts General Hospital Boston, Massachusetts

Textbook of Endocrinology

WILLIAMS

12th EDITION

1600 John F. Kennedy Blvd. Ste 1800 Philadelphia, PA 19103-2899

WILLIAMS TEXTBOOK OF ENDOCRINOLOGY ISBN: 978-1-4377-0324-5 Copyright © 2011, 2008, 2003, 1998, 1992, 1985, 1981, 1974, 1968, 1962, 1955 by Saunders, an imprint of Elsevier Inc. All rights reserved. Copyright 1950 by Saunders, an imprint of Elsevier Inc. Copyright renewed 1990 by A.B. Williams, R.I. Williams Copyright renewed 1983 by William H. Daughaday Copyright renewed 1978 by Robert H. Williams No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Details on how to seek permission, further information about the Publisher’s permissions policies and our arrangements with organizations such as the Copyright Clearance Center and the Copyright Licensing Agency, can be found at our website: www.elsevier.com/permissions. This book and the individual contributions contained in it are protected under copyright by the Publisher (other than as may be noted herein).

Notices Knowledge and best practice in this field are constantly changing. As new research and experience broaden our understanding, changes in research methods, professional practices, or medical treatment may become necessary. Practitioners and researchers must always rely on their own experience and knowledge in evaluating and using any information, methods, compounds, or experiments described herein. In using such information or methods they should be mindful of their own safety and the safety of others, including parties for whom they have a professional responsibility. With respect to any drug or pharmaceutical products identified, readers are advised to check the most current information provided (i) on procedures featured or (ii) by the manufacturer of each product to be administered, to verify the recommended dose or formula, the method and duration of administration, and contraindications. It is the responsibility of practitioners, relying on their own experience and knowledge of their patients, to make diagnoses, to determine dosages and the best treatment for each individual patient, and to take all appropriate safety precautions. To the fullest extent of the law, neither the Publisher nor the authors, contributors, or editors, assume any liability for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions, or ideas contained in the material herein. Library of Congress Cataloging-in-Publication Data Williams textbook of endocrinology.—12th ed. / Shlomo Melmed ... [et al.].    p. ; cm.   Textbook of endocrinology   Includes bibliographical references and index.   ISBN 978-1-4377-0324-5 (hardcover : alk. paper)  1.  Endocrinology.  2.  Endocrine glands— Diseases.  I.  Melmed, Shlomo.  II.  Williams, Robert Hardin.  III.  Title: Textbook of endocrinology.   [DNLM:  1.  Endocrine Glands.  2.  Endocrine System Diseases. WK 100]   RC648.T46 2011   616.4—dc22 2011004321 Acquisitions Editor: Pamela Hetherington Developmental Editor: Joan Ryan Publishing Services Manager: Patricia Tannian Team Manager: Radhika Pallamparthy Senior Project Manager: Sharon Corell Project Manager: Jayavel Radhakrishnan Design Direction: Louis Forgione Printed in the United States of America Last digit is the print number:  9  8  7  6  5  4  3  2  1 

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CONTRIBUTORS

John C. Achermann, MD, PhD Wellcome Trust Senior Fellow in Clinical Science UCL Institute of Child Health Honorary Consultant in Pediatric Endocrinology Great Ormond Street Hospital NHS Trust London, UK

Lloyd P. Aiello, MD, PhD Associate Professor Harvard Medical School Director Beetham Eye Institute, Joslin Diabetes Center Boston, Massachusetts, USA

Andrew Arnold, MD Murray-Heilig Chair in Molecular Medicine Professor of Medicine Chief, Division of Endocrinology and Metabolism University of Connecticut School of Medicine Farmington, Connecticut, USA

Jennifer M. Barker, MD Assistant Professor of Pediatrics University of Colorado Denver Division of Pediatric Endocrinology The Children’s Hospital Aurora, Colorado, USA

Rosemary Basson, MD, FRCP (UK) Clinical Professor Psychiatry University of British Columbia Director, Sexual Medicine Program Psychiatry Vancouver General Hospital Vancouver, British Columbia, Canada

Shalender Bhasin, MD Boston University School of Medicine Boston Medical Center Boston, Massachusetts, USA

Andrew J.M. Boulton, MD, FRCP Professor of Medicine University of Manchester Consultant Physician Manchester Royal Infirmary Manchester, UK

Glenn D. Braunstein, MD Professor and Chairman Department of Medicine The James R. Klinenberg, MD, Chair in Medicine Cedars-Sinai Medical Center Los Angeles, California, USA

William J. Bremner, MD, PhD Professor and Chair Department of Medicine Robert G. Petersdorf Endowed Chair University of Washington School of Medicine Chair Department of Medicine University of Washington Medical Center Seattle, Washington, USA

vi    Contributors

Gregory A. Brent, MD Professor Departments of Medicine and Physiology David Geffen School of Medicine at UCLA Chair, Department of Medicine VA Greater Los Angeles Healthcare System Los Angeles, California, USA

F. Richard Bringhurst, MD Associate Professor Medicine Harvard Medical School Physician Medical Service Massachusetts General Hospital Boston, Massachusetts, USA

Michael Brownlee, MD Anita and Jack Saltz Chair in Diabetes Research and Professor Medicine and Pathology Associate Director for Biomedical Sciences Einstein Diabetes Research Center Albert Einstein College of Medicine Bronx, New York, USA

Serdar E. Bulun, MD Northwestern Memorial Hospital University of Illinois Hospital Chicago, Illinois, USA

Charles F. Burant, MD, PhD Professor of Internal Medicine University of Michigan Ann Arbor, Michigan, USA

John B. Buse, MD, PhD Professor of Medicine Chief, Division of Endocrinology UNC School of Medicine Chapel Hill, North Carolina, USA

David A. Bushinsky, MD Professor Medicine, Pharmacology, and Physiology University of Rochester School of Medicine Rochester, New York, USA

Ernesto Canalis, MD Professor of Medicine University of Connecticut School of Medicine Farmington, Connecticut, USA Director of Research St. Francis Hospital and Medical Center Hartford, Connecticut, USA

Christin Carter-Su, PhD Professor Molecular and Integrative Physiology University of Michigan Medical School Associate Director Michigan Diabetes Research and Training Center University of Michigan Ann Arbor, Michigan, USA

Contributors    vii

Roger D. Cone, PhD Professor and Chairman Molecular Physiology and Biophysics Vanderbilt University School of Medicine Nashville, Tennessee, USA

David W. Cooke, MD Associate Professor Pediatries Johns Hopkins University School of Medicine Baltimore, Maryland, USA

Mark E. Cooper, MB, BS, PhD Director JDRF Centre for Diabetic Complications Professor of Medicine and Immunology Monash University Melbourne, Victoria, Australia

Philip E. Cryer, MD Irene E. and Michael M. Karl Professor of Endocrinology and Metabolism in Medicine Washington University in St. Louis Physician Barnes-Jewish Hospital St. Louis, Missouri, USA

Gilbert H. Daniels, MD Professor of Medicine Harvard Medical School Co-Director Thyroid Clinic Medical Director Endocrine Tumor Center Thyroid Unit, Department of Medicine and Cancer Center Massachusetts General Hospital Boston, Massachusetts, USA

Mehul T. Dattani, FRCPCH, FRCP, MD Professor and Head of Pediatric Endocrinology Developmental Endocrinology Research Group, Clinical and Molecular Genetics Unit UCL Institute of Child Health Consultant and Lead in Adolescent Endocrinology University College London Hospitals London, United Kingdom

Terry F. Davies, MB.BS, MD, FRCP, FACE Florence and Theodore Baumritter Professor of Medicine Attending Physician Mount Sinai School of Medicine New York, New York, USA

Marie B. Demay, MD Professor of Medicine Harvard Medical School Physician Endocrine Unit Department of Medicine Massachusetts General Hospital Boston, Massachusetts, USA

Sara A. DiVall, MD Assistant Professor Pediatries Johns Hopkins University Baltimore, Maryland, USA

viii    Contributors

Daniel J. Drucker, MD Professor of Medicine University of Toronto Toronto, Ontario, Canada

George S. Eisenbarth, MD, PhD Executive Director Professor of Pediatrics Medicine and Immunology Barbara Davis Center for Childhood Diabetes University of Colorado at Denver Professor University of Colorado Hospital Professor, Pediatrics The Children’s Hospital Aurora, Colorado, USA

Joel K. Elmquist, DVM, PhD Professor and Director Division of Hypothalamic Research Internal Medicine and Pharmacology University of Texas Southwestern Medical Center at Dallas Dallas, Texas, USA

Elisa Fabbrini, MD, PhD Research Assistant Professor Center for Human Nutrition Washington University School of Medicine St. Louis, Missouri, USA

Sebastiano Filetti, MD Professor of Internal Medicine Department of Internal Medicine Sapienza Universita’ di Roma Chief, Internal Medicine Department of Internal Medicine Policlinico Umberto I Rome, Italy

Delbert A. Fisher, MD Professor Emeritus Pediatrics and Medicine David Geffen School of Medicine at UCLA Los Angeles, California, USA

Ezio Ghigo, MD Professor of Endocrinology Department of Internal Medicine University of Turin Faculty of Medicine Chief, Division of Endocrinology, Diabetology, and Metabolism Department of Internal Medicine University Hospital Turin, Italy

Anne C. Goldberg, MD Associate Professor of Medicine Division of Endocrinology, Metabolism, and Lipid Research Department of Internal Medicine Washington University School of Medicine St. Louis, Missouri, USA

Ira J. Goldberg, MD Dickinson Richards Jr. Professor Chief, Division of Preventive Medicine and Nutrition Columbia University College of Physicians and Surgeons New York, New York, USA

Contributors    ix

Peter A. Gottlieb, MD Assistant Professor of Pediatrics and Medicine The Children’s Hospital University of Colorado Health Science Center Barbara Davis Center for Childhood Diabetes Aurora, Colorado, USA

Steven K. Grinspoon, MD Professor of Medicine Harvard Medical School Director Neuroendocrine Clinical Center Massachusetts General Hospital Boston, Massachusetts, USA

Melvin M. Grumbach, MD, DM Edward B. Shaw Professor of Pediatrics and Emeritus Chairman Department of Pediatrics University of California, San Francisco Attending Physician Pediatrics University of California, San Francisco Medical Center University of California, San Francisco Children’s Hospital San Francisco, California, USA

Joel F. Habener, MD Professor of Medicine Laboratory of Molecular Endocrinology Massachusetts General Hospital Boston, Massachusetts, USA

Ian D. Hay, MB, PhD, FACE, FACP, FRCP (E,G & l), FRCPI (Hon) Professor of Medicine and Dr. R.F. Emslander Professor of Endocrinology Research Division of Endocrinology and Internal Medicine Mayo Clinic College of Medicine Rochester, Minnesota, USA

Martha Hickey, BA (Hons), MSc, MBChB, MRCOG, FRANZCOG, MD Professor of Gynaecology School of Women’s and Infants’ Health University of Western Australia Perth, Western Australia, Australia

Peter C. Hindmarsh, BSc, MD, FRCP, FRCPCH Professor of Paediatric Endocrinology Developmental Endocrinology Research Group Institute of Child Health University College, London Hon. Consultant Paediatric Endocrinologist Great Ormond Street Hospital for Children Hon. Consultant Paediatric Endocrinologist University College London Hospitals London, United Kingdom

Ken Ho, MD, FRACP, FRCP (UK) Professor of Medicine University of Queensland Chair Centres for Health Research Princess Alexandra Hospital Brisbane, Queensland, Australia

Ieuan A. Hughes, MA, MD, FRCP, FRCP(C), FRCPCH F Med Sci Professor of Paediatrics University of Cambridge Honorary Consultant Paedaitrician Cambridge University Hospitals NHS Foundation Trust Cambridge, United Kingdom

x    Contributors

Andrew M. Kaunitz, MD Professor and Associate Chairman Obstetrics and Gynecology University of Florida College of Medicine, Jacksonville Jacksonville, Florida, USA

George G. Klee, MD, PhD Professor of Laboratory Medicine and Pathology Mayo Clinic College of Medicine Consultant Laboratory Medicine and Pathology Saint Mary’s Hospital Methodist Hospital Mayo Clinic Rochester, Minnesota, USA

Samuel Klein, MD William H. Danforth Professor of Medicine and Nutritional Science Center for Human Nutrition Washington University School of Medicine William H. Danforth Professor of Medicine and Nutritional Science Geriatrics and Nutritional Science Barnes-Jewish Hospital St. Louis, Missouri, USA

David Kleinberg, MD Chief of Endocrinology Veterans Administration Medical Center Department of Medicine New York University New York, New York, USA

Nils P. Krone, MD Wellcome Trust Clinician Scientist Fellow School of Clinical and Experimental Medicine University of Birmingham Consultant Paediatric Endocrinologist Birmingham Children’s Hospital Birmingham, United Kingdom

Henry M. Kronenberg, MD Professor of Medicine Harvard Medical School Chief, Endocrine Unit Massachusetts General Hospital Boston, Massachusetts, USA

Rohit N. Kulkarni, MD, PhD Investigator Cellular and Molecular Physiology Joslin Diabetes Center Assistant Professor of Medicine Department of Medicine Harvard Medical School Boston, Massachusetts, USA

Steven W.J. Lamberts, MD, PhD Professor Erasmus Medical Center Rotterdam, Neatherlands

Fabio Lanfranco, MD, PhD Division of Endocrinology, Diabetology, and Metabolism Department of Internal Medicine University of Turin Turin, Italy

Contributors    xi

P. Reed Larsen, MD, FACP, FRCP Professor of Medicine Harvard Medical School Senior Physician and Chief Thyroid Section Division of Endocrinology, Diabetes, and Metabolism Brigham and Women’s Hospital Boston, Massachusetts, USA

Mitchell A. Lazar, MD, Ph.D. Professor of Medicine and Genetics Chief, Division of Endocrinology, Diabetes, and Metabolism University of Pennsylvania Philadelphia, Pennsylvania, USA

Joseph A. Lorenzo, MD Professor of Medicine University of Connecticut Health Center Attending Physician John Dempsey Hospital Farmington, Connecticut, USA

Malcolm J. Low, MD, PhD Professor of Physiology and Internal Medicine Department of Molecular and Integrative Physiology University of Michigan School of Medicine Ann Arbor, Michigan, USA

Susan J. Mandel, MD, MPH Professor of Medicine and Radiology and Associate Chief Division of Endocrinology, Diabetes, and Metabolism University of Pennsylvania School of Medicine Director, Clinical Endocrinology and Diabetes University of Pennsylvania Health System Philadelphia, Pennsylvania, USA

Stephen J. Marx, MD Branch Chief and Section Chief Metabolic Diseases Branch and Genetics and Endocrinology Section National Institute of Diabetes, Digestive, and Kidney Diseases Bethesda, Maryland, USA

Alvin M. Matsumoto, MD Professor, Department of Medicine University of Washington School of Medicine Associate Director Geriatric Research, Education, and Clinical Center Director, Clinical Research Unit VA Puget Sound Health Care System Seattle, Washington, USA

Shlomo Melmed, MD Professor of Medicine Senior Vice President and Dean of the Faculty Cedars-Sinai Medical Center Los Angeles, California, USA

Rebeca D. Monk, MD Associate Professor of Medicine Nephrology Program Director, Nephrology Fellowship University of Rochester Medical Center Rochester, New York, USA

xii    Contributors

Robert D. Murray, MD Consultant Endocrinologist and Honorary Senior Lecturer Department of Endocrinology Leads Teaching Hospitals NHS Trust Leeds, United Kingdom

Richard W. Nesto, MD Northeast Health System Beverly Hospital Beverly, Massachusetts, USA Lahey Clinic Burlington, Massachusetts, USA

Kjell Oberg, MD, PhD Professor Department of Endocrine Oncology Uppsala University Uppsala, Sweden

Kenneth S. Polonsky, MD Richard T. Crane Distinguished Service Professor Dean of the Division of the Biological Sciences and the Pritzker School of Medicine Executive Vice President of Medical Affairs The University of Chicago Chicago, Illinois, USA

Sally Radovick, MD Johns Hopkins University School of Medicine Johns Hopkins Hospital Baltimore, Maryland, USA

Lawrence G. Raisz, MD Board of Trustees Distinguished Professor of Medicine, Emeritus University of Connecticut Health Center Farmington, Connecticut, USA

Alan G. Robinson, MD Associate Vice Chancellor Medical Sciences, Executive Associate Dean, and Professor of Medicine David Geffen School of Medicine at UCLA Los Angeles, California, USA

Johannes A. Romijn, MD, PhD Professor Endocrinology Leiden University Medical Center Leiden, Netherlands

Domenico Salvatore, MD, PhD Associate Professor of Endocrinology Department of Molecular and Clinical Endocrinology and Oncology University of Naples “Federico II” Naples, Italy

Contributors    xiii

Martin-Jean Schlumberger, MD Professor of Oncology Université Paris Sud. Chair Nuclear Medicine and Endocrine Oncology Institut Gustave Roussy Villejuif, France

Clay F. Semenkovich, MD Herbert S. Gasser Professor Endocrinology, Metabolism, and Lipid Research Washington University St. Louis, Missouri, USA

Allen M. Spiegel, MD The Marilyn and Stanley M. Katz Dean Albert Einstein College of Medicine Bronx, New York, USA

Paul M. Stewart, MD FRCP, FMedSci Dean of Medicine University of Birmingham Consultant Endocrinologist Queen Elizabeth Hospital Birmingham, United Kingdom

Christian J. Strasburger, MD Professor of Medicine Chief Division of Clinical Endocrinology Department of Medicine, CCM Charité Universitätsmedizin Berlin, Germany

Dennis M. Styne, MD Professor and Rimsey Chair of Pediatric Endocrinology University of California Davis Sacramento, California, USA

Simeon I. Taylor, MD, PhD Vice President Discovery Biology Pharmaceutical Research Institute Bristol-Myers Squibb Hopewell, New Jersey, USA

Adrian Vella, MD, FRCP (Edin.) Professor Division of Endocrinology & Metabolism Mayo Clinic Rochester, Minnesota, USA

Joseph G. Verbalis, MD Professor Medicine and Physiology Georgetown University Chief, Endocrinology and Metabolism Georgetown University Hospital Washington, DC, USA

xiv    Contributors

Aaron I. Vinik, MD, PdD FCP, MACP Professor of Medicine, Pathology, Neurobiology Director of Research and Neuroendocrine Unit Eastern Virginia Medical School, Strelitz Diabetes Center Norfolk, Virginia, USA

Samuel Wells, Jr., MD Senior Investigator Medical Qncology Branch National Cancer Institute, NIH Bethesda, Maryland, USA

William F. Young, Jr., MD, MSc Tyson Family Endocrinology Clinical Professor in Honor of Vahab Fatourechi, MD Professor of Medicine Division of Endocrinology, Diabetes, Metabolism, and Nutrition Mayo Clinic Rochester, Minnesota, USA

PREFACE Welcome to the twelfth edition of Williams Textbook of Endocrinology. Robert Williams inaugurated this enduring textbook more than 50 years ago, and the goals have remained essentially unchanged (i.e., to publish “a condensed and authoritative discussion of the management of clinical endocrinopathies based upon the application of fundamental information obtained from chemical and physiological investigation”). Of course, today we would add results of cellular and genomic investigation, as well as the wealth of clinical trial data, as aids in clinical management. The immense and often overwhelming body of new information from multiple disciplines, in fact, makes this synthetic endeavor more relevant than ever to help guide endocrinologists in the care of their patients. To encourage the goal of both highest quality scientific rigor and knowledge synthesis, we continue to ask the most distinguished authors to synthesize entire areas of clinical endocrine science. The mandate for concise yet authoritative and comprehensive presentations acknowledges both the time pressures on today’s physicians and the desire to make the text affordable and easily navigated. This edition has involved extensive revisions of the previous text, and 22 new authors have joined our expert

faculty. A uniform style facilitates identification and ready use of clinical algorithms. We express our deep gratitude to the co-workers in our offices: Anita Nichols, Lynn Moulton, Grace Labrado, Louise Ishibashi, and Sherri Turner, whose energetic efforts have made this work possible. We also thank our colleagues at Elsevier—Joan Ryan, Pamela Hetherington, and Dolores Meloni—who skillfully navigated the dynamic world of medical publishing while assuring achievement of our goals. Their efforts have been essential in ensuring the successful publication of this high-quality textbook, which has become the classic text for all professionals engaged in caring for patients with endocrine disorders. Finally, we would like to recognize and congratulate Dr. Melvin Grumbach for his outstanding contributions to Williams Textbook of Endocrinology, beginning with the fourth edition that was published in 1968. Shlomo Melmed Kenneth S. Polonsky P. Reed Larsen Henry M. Kronenberg

The Evolutionary Perspective,  3 Endocrine Glands,  5 Transport of Hormones in Blood,  6 Target Cells as Active Participants,  7 Control of Hormone Secretion,  8 Hormone Measurement,  10 Endocrine Diseases,  11 Therapeutic Strategies,  12

CHAPTER CHAPTER 1  Principles of Endocrinology

HENRY M. KRONENBERG  •  SHLOMO MELMED  •  P. REED LARSEN  •  KENNETH S. POLONSKY

Roughly 100 years ago, Starling coined the term hormone to describe secretin, a substance secreted by the small intestine into the bloodstream to stimulate pancreatic secretion. In his Croonian Lectures, Starling considered the endocrine and nervous systems as two distinct mechanisms for coordination and control of organ function. Thus, endocrinology found its first home in the discipline of mammalian physiology. Work over the next several decades by biochemists, physiologists, and clinical investigators led to the characterization of many hormones secreted into the bloodstream from discrete glands or other organs. Investigations showed that diseases such as hypothyroidism and diabetes could be treated successfully, for the first time, by replacing specific hormones. These initial triumphs formed the foundation of the clinical specialty of endocrinology. Advances in cell biology, molecular biology, and genetics over the ensuing years began to explain the mechanisms of endocrine diseases and of hormone secretion and action. Even though these advances have embedded endocrinology in the framework of molecular cell biology, they have not changed the essential subject of endocrinology— the signaling mechanisms that coordinate and control the functions of multiple organs and processes. Herein we survey the general themes and principles that underpin the

diverse approaches used by clinicians, physiologists, biochemists, cell biologists, and geneticists to understand the endocrine system.

THE EVOLUTIONARY PERSPECTIVE Hormones can be defined as chemical signals secreted into the bloodstream that act on distant tissues, usually in a regulatory fashion. Hormonal signaling represents a special case of the more general process of signaling between cells. Even unicellular organisms, such as baker’s yeast, Saccharomyces cerevisiae, secrete short peptide mating factors that act on receptors of other yeast cells to trigger mating between the two cells. These receptors resemble the ubiquitous family of mammalian seven-transmembrane spanning receptors that respond to ligands as diverse as photons and glycoprotein hormones. Because these yeast receptors trigger activation of heterotrimeric G proteins just as mammalian receptors do, this conserved signaling pathway must have been present in the common ancestor of yeast and humans. Signals from one cell to adjacent cells—so-called paracrine signals—often trigger cellular responses that use the same molecular pathways used by hormonal signals. For 3

4   Principles of Endocrinology example, the sevenless receptor controls the differentiation of retinal cells in the Drosophila eye by responding to a membrane-anchored signal from an adjacent cell. Sevenless is a membrane-spanning receptor with an intracellular tyrosine kinase domain that signals in a way that closely resembles the signaling by hormone receptors such as the insulin receptor tyrosine kinase. Because paracrine factors and hormones can share signaling machinery, it is not surprising that hormones can, in some settings, act as paracrine factors. Testosterone, for example, is secreted into the bloodstream but also acts locally in the testes to control spermatogenesis. Insulin-like growth factor 1 (IGF1) is a polypeptide hormone that is secreted into the bloodstream from the liver and other tissues, but it is also a paracrine factor made locally in most tissues to control cell proliferation. Furthermore, one receptor can mediate actions of a hormone and a paracrine factor, such as parathyroid hormone (PTH) and parathyroid hormone–related protein. In some cases, the paracrine actions of “hormones” have functions quite unrelated to hormonal functions. For example, macrophages synthesize the active form of vitamin D, 1,25-dihydroxyvitamin D3 or calcitriol, which can then bind to vitamin D receptors in the same cells and stimulate production of antimicrobial peptides.1 The vitamin D 1α-hydroxylase responsible for activating 25hydroxyvitamin D (calcidiol) is synthesized in multiple tissues in which it has functions not apparently related to the calcium homeostatic actions of calcitriol. One can

speculate that the hormonal actions of vitamin D might have evolved well after the paracrine vitamin D apparatus provided the raw materials for the hormonal system. Target cells respond similarly to signals that reach them from the bloodstream (hormones) or from the cell next door (paracrine factors); the cellular response machinery does not distinguish the sites of origin of signals. The shared final common pathways used by hormonal and paracrine signals should not, however, obscure important differences between hormonal and paracrine signaling systems (Fig.1-1). Paracrine signals do not travel very far; consequently, the specific site of origin of a paracrine factor determines where it will act and provides specificity to that action. When the paracrine factor bone morphogenic protein 4 (BMP4) is secreted by cells in the developing kidney, BMP4 regulates the differentiation of renal cells; when the same factor is secreted by cells in bone, it regulates bone formation. Thus, the site of origin of BMP4 determines its physiologic role. In contrast, because hormones are secreted into the bloodstream, their sites of origin are often divorced from their functions. There is nothing about thyroid hormone function, for example, that requires that the thyroid gland be in the neck. Because the specificity of paracrine factor action is so dependent on its precise site of origin, elaborate mechanisms have evolved to regulate and constrain the diffusion of paracrine factors. Paracrine factors of the hedgehog family, for example, are covalently bound to cholesterol to constrain the diffusion of these molecules in the

Regulation of signaling: endocrine Source: gland • No contribution to specificity of target • Synthesis/secretion

Distribution: blood stream • Universal — almost • Importance of dilution

Non-target organ • Metabolism

Target cell • Receptor: source of specificity • Responsiveness: Number of receptors Downstream pathways Other ligands Metabolism of ligand/receptor All often regulated by ligand

Regulation of signaling: paracrine Source: adjacent cell • Major determinant of target • Synthesis/secretion

• • • •

Distribution: matrix Diffusion distance Binding proteins: BMP, IGF Proteases Matrix components

Target cell • Receptor: Specificity and sensitivity Diffusion barrier Determinant of gradient • Induced inhibitory pathways, ligands, and binding proteins

Figure 1-1 Comparison of determinants of endocrine and paracrine signaling.

Principles of Endocrinology   5

extracellular milieu. Most paracrine factors interact with binding proteins that block their action and control their diffusion. For example, chordin, noggin, and many other distinct proteins bind to various members of the BMP family to regulate their action. Proteases such as tolloid then destroy the binding proteins at specific sites to liberate BMPs so that they can act on appropriate target cells. Hormones have rather different constraints. Because they diffuse throughout the body, they must be synthesized in enormous amounts relative to the amounts of paracrine factors needed at specific locations. This synthesis usually occurs in specialized cells designed for that specific purpose. Hormones must then be able to travel in the bloodstream and diffuse in effective concentrations into tissues. Lipophilic hormones, for example, bind to soluble proteins that allow them to travel in the aqueous medium of blood at relatively high concentrations. The ability of hormones to diffuse through the extracellular space means that the local concentration of a hormone at target sites will rapidly decrease when glandular secretion of the hormone stops. Because hormones diffuse throughout extracellular fluid quickly, hormonal metabolism can occur in specialized organs (e.g., liver, kidney) in a manner that determines the effective hormone concentration in other tissues. In summary, paracrine factors and hormones use several distinct strategies to control their biosynthesis, sites of action, transport, and metabolism. These differing strategies probably explain partly why a hormone such as IGF1, unlike its close relative, insulin, has multiple binding proteins that control its action in tissues. IGF1 exhibits a double life—it is both a hormone and a paracrine factor. Presumably, the local actions of IGF1 mandate an elaborate binding protein apparatus to enable appropriate hormone signaling. All of the major hormonal signaling programs—G protein–coupled receptors, tyrosine kinase receptors, serine/threonine kinase receptors, ion channels, cytokine receptors, and nuclear receptors—are also used by paracrine factors. In contrast, several paracrine signaling programs appear to be used only by paracrine factors and not by hormones. For example, Notch receptors respond to membrane-based ligands to control cell fate, but no bloodborne ligands are known to use Notch-type signaling. Perhaps the intracellular strategy used by Notch, which involves cleavage of the receptor and subsequent nuclear actions of the receptor’s cytoplasmic portion, is too inflexible to serve the purposes of hormones. The analyses of the complete genomes of multiple bacterial species, the yeast S. cerevisiae, the fruit fly Drosophila melanogaster, the worm Caenorhabditis elegans, the plant Arabidopsis thaliana, humans, and many other species have allowed a comprehensive view of the signaling machinery used by various forms of life. As noted earlier, S. cerevisiae uses G protein–linked receptors; this organism, however, lacks tyrosine kinase receptors and nuclear receptors that resemble the estrogen/thyroid receptor family. In contrast, the worm and fly share with humans the use of each of these signaling pathways, although with substantial variation in the number of genes committed to each pathway. For example, the Drosophila genome encodes 20 nuclear receptors, the C. elegans genome 270, and the human genome (tentatively) more than 50. These patterns suggest that ancient multicellular animals must have already established the signaling systems that are the foundation of the endocrine system as we know it in mammals. Even before the sequencing of the human genome was accomplished, sequence analyses had made clear that many receptor genes are found in mammalian genomes for

which no clear ligand or function is known. The analyses of these “orphan” receptors have succeeded in broadening the current understanding of hormonal signaling. For example, the orphan liver X receptor, LXR, was found during searches for unknown nuclear receptors. Subsequent experiments showed that oxygenated derivatives of cholesterol are the ligands for LXR, which regulates genes involved in cholesterol and fatty acid metabolism.2 LXR and many other examples raise the question of what constitutes a hormone. The classic view of hormones is that they are synthesized in discrete glands and have no function other than activating receptors on cell membranes or in the nucleus. In contrast, cholesterol, which is converted in cells to oxygenated derivatives that activate the LXR receptor, uses a hormonal strategy to regulate its own metabolism. Other orphan nuclear receptors similarly respond to ligands such as bile acids and fatty acids. These “hormones” have important metabolic roles quite separate from their signaling properties, although the hormone-like signaling serves to allow regulation of the metabolic function. The calciumsensing receptor is an example from the G protein–linked receptor family that responds to a nonclassic ligand, ionic calcium. Calcium is released into the bloodstream from bone, kidney, and intestine and acts on the calcium-sensing receptors on parathyroid cells, renal tubular cells, and other cells to coordinate cellular responses to calcium. Therefore, many important metabolic factors have taken on hormonal properties as part of a regulatory strategy.

ENDOCRINE GLANDS Hormone formation may occur either in localized collections of specific cells, the endocrine glands, or in cells that have additional roles. Many protein hormones, such as growth hormone (GH), PTH, prolactin, insulin, and glucagon, are produced in dedicated cells by standard protein synthetic mechanisms common to all cells. These secretory cells usually contain specialized secretory granules designed to store large amounts of hormone and to release the hormone in response to specific signals. Formation of small hormone molecules begins with commonly found precursors, usually located in specific glands such as the adrenals, gonads, or thyroid. In the case of the steroid hormones, the precursor is cholesterol, which is modified by various hydroxylations, methylations, and demethylations to form glucocorticoids, androgens, estrogens, and their biologically active derivatives. However, not all hormones are formed in dedicated and specialized endocrine glands. For example, the protein hormone, leptin, which regulates appetite and energy expenditure, is formed in adipocytes, providing a specific signal that reflects the nutritional state to the central nervous system. The cholesterol derivative, 7-dehydrocholesterol, the precursor of vitamin D, is produced in skin keratinocytes by a photochemical reaction. In the unique enteroendocrine hormonal system, peptide hormones that regulate metabolic and other responses to oral nutrients are produced and secreted by specialized endocrine cells scattered throughout the intestinal epithelium. Thyroid hormone synthesis occurs by means of a unique pathway. The thyroid cell synthesizes a 660,000-kd homodimer, thyroglobulin, which is then iodinated at specific iodotyrosines. Certain of these “couple” to form the iodothyronine molecule within thyroglobulin, which is then stored in the lumen of the thyroid follicle. For this to occur,

6   Principles of Endocrinology the thyroid cell must concentrate the trace quantities of iodide from the blood and oxidize it via a specific peroxidase. Release of thyroxine (T4) from the thyroglobulin requires its phagocytosis and cathepsin-catalyzed digestion by the same cells. Hormones are synthesized in response to biochemical signals generated by various modulating systems. Many of these systems are specific to the effects of the hormone product. For example, PTH synthesis is regulated by the concentration of ionized calcium, and insulin synthesis is regulated by the concentration of glucose. For others, such as gonadal, adrenal, and thyroid hormones, control of hormone synthesis is achieved by the hormonostatic function of the hypothalamic-pituitary axis. Cells in the hypothalamus and pituitary monitor the circulating hormone concentration and secrete tropic hormones, which activate specific pathways for hormone synthesis and release. Typical examples are luteinizing hormone (LH), folliclestimulating hormone (FSH), thyroid-stimulating hormone (TSH), and adrenocorticotropic hormone (ACTH). These trophic hormones increase rates of hormone synthesis and secretion, and they may induce target cell division, resulting in enlargement of the various target glands. For example, in hypothyroid individuals living in iodinedeficient areas of the world, TSH secretion causes a marked hyperplasia of thyroid cells. In such regions, the thyroid gland may be 20 to 50 times its normal size. Adrenal hyperplasia occurs in patients with genetic deficiencies in cortisol formation. Hypertrophy and hyperplasia of parathyroid cells, in this case initiated by an intrinsic response to the stress of hypocalcemia, occurs in patients with renal insufficiency or calcium malabsorption. Hormones may be fully active when they are released into the bloodstream (e.g., GH, insulin), or they may require activation in specific cells to produce their biologic effects. These activation steps are often highly regulated. For example, the T4 released from the thyroid cell is a prohormone that must undergo a specific deiodination to form the active 3,5,3′-triiodothyronine (T3). This deiodination reaction can occur in target tissues (e.g., in the central nervous system); in the thyrotrophs, where T3 provides feedback regulation of TSH production; or in hepatic and renal cells, from which it is released into the circulation for uptake by all tissues. A similar postsecretory activation step, catalyzed by a 5αα-reductase, causes tissue-specific activation of testosterone to dihydrotestosterone in target tissues including the male urogenital tract and genital skin, as well as in liver. Vitamin D undergoes hydroxylation at the 25 position in the liver and at the 1 position in the kidney. Both hydroxylations must occur to produce the active hormone, calcitriol. The activity of 1α-hydroxylase, but not that of 25-hydroxylase, is stimulated by PTH and reduced plasma phosphate but inhibited by calcium, calcitriol, and fibroblast growth factor 23 (FGF23). Hormones are synthesized as required on a daily, hourly, or minute-to-minute basis with minimal storage, but there are significant exceptions. One is the thyroid gland, which contains enough stored hormone to last for about 2 months. This permits a constant supply despite significant variations in the availability of iodine. However, if iodine deficiency is prolonged, the normal reservoirs of T4 can be depleted. The various feedback signaling systems already described enable the hormonal homeostasis that is characteristic of virtually all endocrine systems. Regulation may include the central nervous system or local signal recognition mechanisms in the glandular cells, such as the calcium-sensing receptor of the parathyroid cell. Superimposed, centrally

programmed increases and decreases in hormone secretion or activation also occur through neuroendocrine pathways. Examples include the circadian variation in secretion of ACTH that directs the synthesis and release of cortisol. The monthly menstrual cycle exemplifies a system with much longer periodicity that requires a complex synergism between central and peripheral axes of the endocrine glands. Disruption of hormonal homeostasis due to glandular or central regulatory system dysfunction has both clinical and laboratory consequences. Recognition and correction of these effects are the essence of clinical endocrinology.

TRANSPORT OF HORMONES IN BLOOD Protein hormones and some small molecules, such as the catecholamines, are water soluble and readily transported via the circulatory system. Others, such as the steroid and thyroid hormones, are almost insoluble in water, and their distribution presents special problems. Such molecules are bound to 50- to 60-kd carrier plasma glycoproteins such as thyroxine-binding globulin (TBG), sex hormone–binding globulin (SHBG), and corticosteroid-binding globulin, as well as to albumin. The ligand-protein complexes serve as reservoirs of these hormones, ensure ubiquitous distribution of their water-insoluble ligands, and protect the small molecules from rapid inactivation or excretion in the urine or bile. Without these proteins, it is unlikely that hydrophobic molecules would be transported much beyond the veins draining the glands in which they are formed. The protein-bound hormones exist in rapid equilibrium with the often minute quantities of hormone in the aqueous plasma. It is this “free” fraction of the circulating hormone that is taken up by the cell. For example, if tracer thyroid hormone is injected into the portal vein in a protein-free solution, it becomes bound to hepatocytes at the periphery of the hepatic sinusoid. When the same experiment is repeated with a protein-containing solution, there is a uniform distribution of tracer hormone throughout the hepatic lobule.3 Despite the very high affinity of some of the binding proteins for their ligands, a specific protein may not be essential for hormone distribution. For example, in humans with a congenital deficiency of TBG, other proteins, namely transthyretin (TTR) and albumin, subsume its role. Because the affinity of these secondary thyroid hormone transport proteins is several orders of magnitude lower than that of TBG, it is possible for the hypothalamic-pituitary feedback system to maintain free thyroid hormone in the normal range at a much lower total hormone concentration. The fact that the level of “free” hormone concentration is normal in subjects with TBG deficiency indicates that it is this free moiety that is defended by the hypothalamicpituitary axis and is the active hormone.4 The availability of gene targeting techniques has allowed specific tests of the physiologic roles of several hormonebinding proteins. For example, mice with targeted inactivation of the vitamin D–binding protein (DBP) have been generated.5 Although the absence of DBP markedly reduces the circulating concentration of vitamin D, the mice are otherwise normal. However, they do show enhanced susceptibility to a vitamin D–deficient diet because of the reduced reservoir of this sterol. In addition, the absence of DBP markedly reduces the half-life of calcidiol by accelerating its hepatic uptake, making the mice less susceptible to vitamin D intoxication.

Principles of Endocrinology   7

In rodents, TTR carries retinol-binding protein and is also the principal thyroid hormone–binding protein. This protein is synthesized in the liver and in choroid plexus. It is the major thyroid hormone–binding protein in the cerebrospinal fluid of both rodents and humans and was previously thought to possibly serve an important role in thyroid hormone transport into the central nervous system. This hypothesis was later disproved by the fact that mice without TTR have normal concentrations of T4 in the brain in addition to free T4 in the plasma.6,7 To be sure, the serum concentrations of vitamin A and total T4 are decreased, but the knockout mice have no signs of vitamin A deficiency or hypothyroidism. Such studies suggest that these proteins primarily serve distributive and reservoir functions. Protein hormones and some small ligands (e.g., catecholamines) produce their effects by interacting with cell surface receptors. Others, such as the steroid and thyroid hormones, must enter the cell to bind to cytosolic or nuclear receptors. In the past, it was thought that much of the transmembrane transport of hormones was passive. Evidence has now demonstrated that specific transporters are involved in cellular uptake of thyroid hormone.8 This may be found to be the case for other small ligands as well, revealing yet another mechanism for ensuring the distribution of a hormone to its site of action. Studies in mice missing megalin, a large cell surface protein in the lowdensity lipoprotein (LDL) receptor family, have suggested that estrogen and testosterone uses megalin to enter certain tissues while still bound to SHBG.9 In this case, therefore,

it is the hormone bound to SHBG, rather than the “free” hormone, that is the active moiety that enters cells. It is unclear how generally this apparent exception to the “free hormone” hypothesis occurs.

TARGET CELLS AS ACTIVE PARTICIPANTS Hormones determine cellular target actions by binding with high specificity to receptor proteins. Whether a peripheral cell is hormonally responsive depends to a large extent on the presence and function of specific and selective hormone receptors. Receptor expression determines which cells will respond as well as the nature of the intracellular effector pathways activated by the hormone signal. Receptor proteins may be localized to the cell membrane, cytoplasm, or nucleus. Broadly, polypeptide hormone receptors are associated with cell membranes, whereas steroid hormones selectively bind soluble intracellular proteins (Fig. 1-2). However, exceptions do occur. For example, epidermal growth factor (EGF) may signal directly to receptors located within the nucleus. Membrane-associated receptor proteins usually consist of extracellular sequences that recognize and bind ligand, transmembrane anchoring hydrophobic sequences, and intracellular sequences that initiate intracellular signaling. Intracellular signaling is mediated by covalent modification and activation of intracellular signaling molecules Progesterone O

O

R

O

O

TF PKA

TFTyr

P

AC LH

P

R

Target gene XTyr

ATP

G

XTyr

O

cAMP

s ss

Figure 1-2  Hormonal signaling by cell surface and intracellular receptors. The receptors for the water-soluble polypeptide hormones, luteinizing hormone (LH), and insulin-like growth factor 1 (IGF-1), are integral membrane proteins located at the cell surface. They bind the hormone-utilizing extracellular sequences and transduce a signal through the generation of second messengers: cyclic adenosine monophosphate (cAMP) for the LH receptor and tyrosinephosphorylated substrates for the IGF-1 receptor. Although effects on gene expression are indicated, direct effects on cellular proteins (e.g., ion channels) are also observed. In contrast, the receptor for the lipophilic steroid hormone, progesterone, resides in the cell nucleus. It binds the hormone and becomes activated and capable of directly modulating target gene transcription.) AC, Adenylate cyclase; G, heterotrimeric G protein; mRNAs, messenger RNAs; PKA, protein kinase A; R, receptor molecule;TF, transcription factor; Tyr, tyrosine found in protein X; X, unknown protein substrate. (Reproduced from Mayo K. Receptors: molecular mediators of hormone action. In: Conn PM, Melmed S, eds. Endocrinology: Basic and Clinical Principles. Totowa, NJ: Humana Press, 1997:11.)

RR

AAAAA

mRNAs

R

Proteins

Biological responses

P

ss

s

IGF-1

8   Principles of Endocrinology (e.g., STAT proteins) or by generation of small molecule second messengers (e.g., cyclic adenosine monophosphate) through activation of heterotrimeric G proteins. The α-, β-, and γ-subunits of these G proteins activate or suppress effector enzymes and ion channels that generate the second messengers. Some of these receptors may in fact exhibit constitutive activity and have been shown to signal in the absence of added ligand. Several growth factors and hormone receptors (e.g., for insulin) behave as intrinsic tyrosine kinases or activate intracellular protein tyrosine kinases. Ligand activation may cause receptor dimerization (e.g., GH) or heterodimerization (e.g., interleukin-6), followed by activation of intracellular phosphorylation cascades. These activated proteins ultimately determine specific nuclear gene expression. Both the number of receptors expressed per cell and their responses are regulated, providing a further level of control for hormone action. Several mechanisms account for altered receptor function. Receptor endocytosis causes internalization of cell surface receptors; the hormonereceptor complex is subsequently dissociated, resulting in abrogation of the hormone signal. Receptor trafficking may then result in recycling back to the cell surface (e.g., as for insulin), or the internalized receptor may undergo lysosomal degradation. Both of these mechanisms, triggered by activation of receptors, effectively lead to impaired hormone signaling by downregulation of these receptors. The hormone signaling pathway may also be downregulated by receptor desensitization (e.g., as for epinephrine); ligand-mediated receptor phosphorylation leads to a reversible deactivation of the receptor. Desensitization mechanisms can be activated by a receptor’s ligand (homologous desensitization) or by another signal (heterologous desensitization), which attenuates receptor signaling in the continued presence of ligand. Receptor function may also be limited by the action of specific phosphatases (e.g., SHP) or by intracellular negative regulation of the signaling cascade (e.g., suppressor of cytokine signaling [SOCS] proteins inhibiting JAK-STAT signaling). Mutational changes in receptor structure can also determine hormone action. Constitutive receptor activation

may be induced by activating mutations (e.g., TSH receptor) leading to endocrine organ hyperfunction, even in the absence of hormone. Conversely, inactivating receptor mutations may lead to endocrine hypofunction (e.g., testosterone receptor, vasopressin receptor). These syndromes are well characterized and are well described in other chapters of this text (Fig. 1-3). The functional diversity of receptor signaling also results in overlapping or redundant intracellular pathways. For example, both GH and cytokines activate JAK-STAT signaling, whereas the distal effects of these stimuli clearly differ. Therefore, despite common signaling pathways, hormones elicit highly specific cellular effects. Tissue or cell-type genetic programs or receptor-receptor interactions at the cell surface (e.g., dopamine D2 hetero-oligomerization with somatotropin release–inhibiting factor [SRIF]) may also confer specific cellular response to a hormone and provide an additive cellular effect.10

CONTROL OF HORMONE SECRETION Anatomically distinct endocrine glands are composed of highly differentiated cells that synthesize, store, and secrete hormones. Circulating hormone concentrations are a function of glandular secretory patterns and hormone clearance rates. Hormone secretion is tightly regulated to attain circulating levels that are most conducive to eliciting the appropriate target tissue response. For example, longitudinal bone growth is initiated and maintained by exquisitely regulated levels of circulating GH: mild GH hypersecretion results in gigantism, and GH deficiency causes growth retardation. Ambient circulating hormone concentrations are not uniform, and the secretion patterns determine appropriate physiologic function. For example, insulin secretion occurs in short pulses elicited by nutrient intake and other signals; gonadotropin secretion is episodic, determined by a hypothalamic pulse generator; and prolactin secretion appears to be relatively continuous with secretory peaks elicited during suckling.

Diseases caused by mutations in G-protein-coupled receptors Condition

Receptor

Inheritance

∆ Function

Retinitis pigmentosa Nephrogenic diabetes insipidus Isolated glucocorticoid deficiency Color blindness Familial precocious puberty Familial hypercalcemia Neonatal severe parathyroidism Dominant form hypocalcemia Congenital hyperthyroidism Resistance to thyroid hormone Hyperfunctioning thyroid adenoma Metaphyseal chondrodysplasia Hirschsprung’s disease

Rhodopsin Vasopressin V2 ACTH Red/green opsins LH Ca2+ sensing Ca2+ sensing Ca2+ sensing TSH TSH TSH PTH-PTHrP Endothelin-B

AD/AR X-linked AR X-linked AD (male) AD AR AD AD AR (comp het) Somatic Somatic Multigenic

Loss Loss Loss Loss Gain Loss Loss Gain Gain Loss Gain Gain Loss

Coat color alteration (E locus, mice) Dwarfism (little locus, mice)

MSH GHRH

AD/AR AR

Loss and gain Loss

Figure 1-3 Diseases caused by mutations in G-protein–coupled receptors. All are human conditions with the exception of the final two entries, which refer to the mouse. Loss of function refers to inactivating mutations of the receptor, and gain of function to activating mutations. ACTH, adrenocorticotropic hormone; AD, autosomal dominant; AR, autosomal recessive; LH, luteinizing hormone; TSH, thyroid-stimulating hormone; PTH-PTHrP, parathyroid hormone and parathyroid hormone–related peptide; MSH, melanocyte-stimulating hormone; GHRH, growth hormone–releasing hormone; FSH, follicle-stimulating hormone. (Reproduced from Mayo K. Receptors: molecular mediators of hormone action. In Conn PM, Melmed S, eds. Endocrinology: Basic and Clinical Principles. Totowa, NJ: Humana Press, 1997:27.)

Principles of Endocrinology   9

Hormone secretion also adheres to rhythmic patterns. Circadian rhythms serve as adaptive responses to environmental signals and are controlled by a circadian timing mechanism.11 Light is the major environmental cue adjusting the endogenous clock. The retinohypothalamic tract entrains circadian pulse generators situated within hypothalamic suprachiasmatic nuclei. These signals subserve timing mechanisms for the sleep-wake cycle and determine patterns of hormone secretion and action. Disturbed circadian timing results in hormonal dysfunction and may also be reflective of entrainment or pulse generator lesions. For example, adult GH deficiency due to a damaged hypothalamus or pituitary is associated with elevations in integrated 24-hour leptin concentrations, decreased leptin pulsatility, and yet preserved circadian rhythm of leptin. GH replacement restores leptin pulsatility, followed by loss of body fat mass.12 Sleep is also an important cue regulating hormone pulsatility. About 70% of overall GH secretion occurs during slow-wave sleep, and increasing age is

associated with declining slow-wave sleep and concomitant decline in GH and elevation of cortisol secretion.13 Most pituitary hormones are secreted in a circadian (daynight) rhythm, best exemplified by the ACTH peaks that occur before 9 a.m., whereas ovarian steroids follow a 28-day menstrual rhythm. Disrupted episodic rhythms are often a hallmark of endocrine dysfunction. For example, loss of circadian ACTH secretion with high midnight cortisol levels is a feature of Cushing’s disease. Hormone secretion is induced by multiple specific biochemical and neural signals. Integration of these stimuli results in net temporal and quantitative secretion of the hormone (Fig. 1-4). For example, signals elicited by hypothalamic hormones (growth hormone–releasing hormone [GHRH], SRIF), peripheral hormones (IGF1, sex steroids, thyroid hormone), nutrients, adrenergic pathways, stress, and other neuropeptides all converge on the somatotroph cell, resulting in the ultimate pattern and quantity of GH secretion. Networks of reciprocal interactions allow for

External/internal environmental signals

Central nervous system

Electric or chemical transmission

Long feedback loop

Short feedback loop

Fast feedback loop

Hypothalamus

Axonal transport

Releasing hormones (ng)

Oxytocin, vasopressin

Adenohypophysis

Neurohypophysis

Anterior pituitary hormones (µg)

Target glands

Release

Uterine contraction Lactation (oxytocin)

Water balance (vasopressin)

Ultimate hormone (µg-mg)

Hormonal response Figure 1-4  Peripheral feedback mechanism and a million-fold amplifying cascade of hormonal signals. Environmental signals are transmitted to the central nervous system, which innervates the hypothalamus, which responds by secreting nanogram amounts of a specific hormone. Releasing hormones are transported down a closed portal system, pass the blood-brain barrier at either end through fenestrations, and bind to specific anterior pituitary cell membrane receptors to elicit secretion of microgram amounts of specific anterior pituitary hormones. These enter the venous circulation through fenestrated local capillaries, bind to specific target gland receptors, trigger release of micrograms to milligrams of daily hormone amounts, and elicit responses by binding to receptors in distal target tissues. Peripheral hormone receptors enable widespread cell signaling by a single initiating environmental signal, thus facilitating intimate homeostatic association with the external environment. Arrows with a black dot at their origin indicate a secretory process. (Reproduced from Normal AW, Litwack G. Hormones, ed 2. New York, NY: Academic Press, 1997:14.)

10   Principles of Endocrinology dynamic adaptation and shifts in environmental signals. These regulatory systems embrace the hypothalamic pituitary and target endocrine glands as well as the adipocyte and lymphocyte. Peripheral inflammation and stress elicit cytokine signals that interface with the neuroendocrine system, resulting in activation of the hypothalamicpituitary axis. The parathyroid and pancreatic secreting cells are less tightly controlled by the hypothalamus, but their functions are tightly regulated by the effects they elicit. For example, PTH secretion is induced when serum calcium levels fall, and the signal for sustained PTH secretion is abrogated by rising calcium levels. Several tiers of control play a part in the ultimate net glandular secretion. First, central nervous system signals, including stress, afferent stimuli, and neuropeptides, signal the synthesis and secretion of hypothalamic hormones and neuropeptides (Fig. 1-5). Four hypothalamicreleasing hormones—GHRH, corticotropin-releasing hormone, thyrotropin-releasing hormone, and gonadotropin-releasing hormone (GnRH)—traverse the hypothalamic portal vessels and impinge upon their respective transmembrane trophic hormone-secreting cell receptors. These distinct cells express GH, ACTH, TSH, and gonadotropins, respectively. In contrast, hypothalamic somatostatin and dopamine suppress secretion of GH, prolactin, and TSH. Trophic hormones also maintain the structural and functional integrity of endocrine organs, including the thyroid and adrenal glands and the gonads. Target hormones, in turn, serve as powerful negative feedback regulators of their respective trophic hormones; they often also suppress secretion of hypothalamic-releasing hormones. In certain circumstances (e.g., during puberty), peripheral sex steroids may positively induce the hypothalamic-pituitary– target gland axis. LH induces ovarian estrogen secretion, which feeds back positively to induce further LH release. Pituitary hormones themselves, in a short feedback loop, may also regulate their own respective hypothalamiccontrolling hormone. Hypothalamic releasing hormones

CNS Inputs

Hypothalamus

Pituitary

Pituitary trophic hormone Target gland

Tier I Hypothalamic hormones

Tier II Paracrine cytokines and growth factors

Tier III Peripheral hormones

Figure 1-5  Model for regulation of anterior pituitary hormone secretion by three tiers of control. Hypothalamic hormones impinge directly on their respective target cells. Intrapituitary cytokines and growth factors regulate tropic cell function through paracrine (and autocrine) control. Peripheral hormones exert negative feedback inhibition on the synthesis and secretion of their respective pituitary trophic hormones. (Reproduced from Ray D, Melmed S. Pituitary cytokine and growth factor expression and action. Endocrine Rev. 1997;18:206-228.)

are secreted in nanogram amounts and have short halflives of a few minutes. Anterior pituitary hormones are produced in microgram amounts and have longer halflives, whereas peripheral hormones can be produced in up to milligram amounts daily, with much longer half-lives. A further level of secretion control occurs within the gland itself. Intraglandular paracrine or autocrine growth peptides serve to autoregulate pituitary hormone secretion, as exemplified by EGF control of prolactin or IGF1 control of GH secretion. Molecules within the endocrine cell may also subserve an intracellular feedback loop. For example, corticotrope SOCS3 induction by gp130-linked cytokines serves to abrogate the ligand-induced JAK-STAT cascade, blocking transcription of pro-opiomelanocortin and subsequent secretion of ACTH. This rapid on-off regulation of ACTH secretion provides a plastic endocrine response to changes in environmental signaling and serves to maintain homeostatic integrity.14 In addition to the central-neuroendocrine interface mediated by hypothalamic chemical signal transduction, the central nervous system directly controls several hormonal secretory processes. Posterior pituitary hormone secretion occurs as direct efferent neural extensions. Postganglionic sympathetic nerves also regulate rapid changes in renin, insulin, and glucagon secretion, and preganglionic sympathetic nerves signal to adrenal medullary cells, eliciting adrenaline release.

HORMONE MEASUREMENT Endocrine function can be assessed by measuring levels of basal circulating hormone, evoked or suppressed hormone, or hormone-binding proteins. Alternatively, peripheral hormone receptor function can be assessed. Meaningful strategies for timing hormonal measurements vary from system to system. In some cases, circulating hormone concentrations can be measured in randomly collected serum samples. This measurement, when standardized for fasting, environmental stress, age, and gender, is reflective of true hormone concentrations only when levels do not fluctuate appreciably. For example, thyroid hormone, prolactin, and IGF1 levels can be accurately assessed in fasting morning serum samples. If hormone secretion is clearly episodic, timed samples may be required over a defined time course to reflect hormone bioavailability. For example, early morning and late evening cortisol measurements are most appropriate. Sampling over 24 hours for GH measurements, with samples collected every 2, 10, or 20 minutes, is expensive and cumbersome, yet may yield valuable diagnostic information. Random sampling may reflect secretion peaks or nadirs, confounding adequate interpretation of results. In general, confirmation of failed glandular function is made by attempting to evoke hormone secretion by recognized stimuli. For example, testing of pituitary hormone reserve may be accomplished by injecting appropriate hypothalamic releasing hormones. Injection of trophic hormones, including TSH and ACTH, evokes specific target gland hormone secretion. Pharmacologic stimuli (e.g., metoclopramide for induction of prolactin secretion) may also be useful to test hormone reserve. In contrast, hormone hypersecretion can be diagnosed by suppressing glandular function. For example, failure to appropriately suppress GH levels after a standardized glucose load implies inappropriate GH hypersecretion. The failure to suppress insulin

Principles of Endocrinology   11

secretion in response to hypoglycemia indicates inappropriate hypersecretion of insulin and should prompt a search for the cause (e.g., an insulin-secreting tumor). Radioimmunoassays use highly specific antibodies unique to a hormone, or hormone fragment, to quantify hormone levels. Enzyme-linked immunosorbent assays (ELISAs) employ enzymes instead of radioactive hormone markers, and enzyme activity reflects hormone concentration. This sensitive technique has allowed ultrasensitive measurements of physiologic hormone concentrations. Hormone-specific receptors may be employed in place of the antibody in a radioreceptor assay.

ENDOCRINE DISEASES Endocrine diseases fall into five broad categories: (1) hormone overproduction, (2) hormone underproduction, (3) altered tissue responses to hormones, (4) tumors of endocrine glands, and a relatively new category exemplified in the thyroid axis, (5) hormone deficiency due to its excessive rate of inactivation caused by overexpression of an endogenous enzyme in a tumor.

Hormone Overproduction Occasionally, hormones are secreted in increased amounts because of genetic abnormalities that cause abnormal regulation of hormone synthesis or release. For example, in glucocorticoid-remediable hyperaldosteronism, an abnormal chromosomal crossover event puts the aldosterone synthetase gene under the control of the ACTH-regulated 11β-hydroxylase gene. More often, diseases of hormone overproduction are associated with an increase in the total number of hormone-producing cells. For example, the hyperthyroidism of Graves’ disease, in which antibodies mimic TSH and activate the TSH receptors on thyroid cells, is associated with a dramatic increase in thyroid cell proliferation, as well as increased synthesis and release of thyroid hormone from each thyroid cell. The increase in thyroid cell number represents a polyclonal expansion of thyroid cells in which large numbers of thyroid cells proliferate in response to an abnormal stimulus. However, most endocrine tumors are not polyclonal expansions but instead represent monoclonal expansions of one mutated cell. Pituitary and parathyroid tumors, for example, are usually monoclonal expansions in which somatic mutations in multiple tumor suppressor genes and protooncogenes occur. These mutations lead to increased proliferation or survival (or both) of the mutant cells. Sometimes, this proliferation is associated with abnormal secretion of hormone from each tumor cell. For example, mutant Gs α-subunit proteins in somatotrophs can lead to both increased cellular proliferation and increased secretion of GH from each tumor cell.

Hormone Underproduction Underproduction of hormone can result from a wide variety of processes, ranging from surgical removal of parathyroid glands during neck surgery, to tuberculous destruction of adrenal glands, to iron deposition in beta cells of islets in hemochromatosis. A frequent cause of destruction of hormone-producing cells is autoimmunity. Autoimmune destruction of beta cells in type 1 diabetes mellitus or of thyroid cells in Hashimoto’s thyroiditis are two of the most common disorders treated by endocrinologists. More

uncommonly, a host of genetic abnormalities can lead to decreased hormone production. These disorders can result from abnormal development of hormone-producing cells (e.g., hypogonadotropic hypogonadism caused by KAL gene mutations), from abnormal synthesis of hormones (e.g., deletion of the GH gene), or from abnormal regulation of hormone secretion (e.g., the hypoparathyroidism associated with activating mutations of the parathyroid cell’s calcium-sensing receptor).

Altered Tissue Responses Resistance to hormones can be caused by a variety of genetic disorders. Examples include mutations in the GH receptor in Laron dwarfism and mutations in the Gsα gene in the hypoparathyroidism of pseudohypoparathyroidism type 1A. The insulin resistance in muscle and liver that is central to the etiology of type 2 diabetes mellitus appears to be polygenic in origin. Type 2 diabetes is also an example of a disease in which end-organ insensitivity is worsened by signals from other organs, in this case by signals originating in fat cells. In other cases, the target organ of hormone action is more directly abnormal, as in the PTH resistance of renal failure. Increased end-organ function can be caused by mutations in signal reception and propagation. For example, activating mutations in TSH, LH, and PTH receptors can cause increased activity of thyroid cells, Leydig cells, and osteoblasts, even in the absence of ligand. Similarly, activating mutations in the Gsα protein can cause precocious puberty, hyperthyroidism, and acromegaly in McCuneAlbright syndrome.

Tumors of Endocrine Glands Tumors of endocrine glands, as mentioned earlier, often result in hormone overproduction. Some tumors of endocrine glands produce little if any hormone but cause disease through local compressive symptoms or metastatic spread. Examples include so-called nonfunctioning pituitary tumors, which are usually benign but can cause a variety of symptoms due to compression on adjacent structures, and thyroid cancer, which can spread throughout the body without causing hyperthyroidism.

Excessive Hormone Inactivation or Destruction Although most enzymes important for endocrine systems activate a prohormone or precursor protein, there are also those whose function is to inactivate the hormone in a physiologically regulated fashion. An example is iodothyronine deiodinase type 3 (D3) which inactivates T3 and T4 by removing an inner-ring iodine atom from the iodothyronine. The products of these reactions, 3,3′diiodothyronine and reverse T3, respectively, are inactive. Several years ago, an infant was identified with hypothyroidism due to a large hepatic hemangioma expressing high amounts of D3. This condition was termed consumptive hypothyroidism because it resulted from inactivation of thyroid hormone at a more rapid rate than it could be produced by the infant’s normal thyroid gland.15,16 The condition has now been identified in a number of infants and even in adults with D3-expressing tumors. In theory, accelerated destruction of other hormones could occur from similar processes, but there are no examples reported to date.

12   Principles of Endocrinology

THERAPEUTIC STRATEGIES In general, hormones are employed pharmacologically for their replacement or suppressive effects. Hormones may also be used for diagnostic stimulatory purposes (e.g., hypothalamic hormones) to evoke target organ responses or to diagnose endocrine hyperfunction by suppressing hormone hypersecretion (e.g., T3). Ablation of endocrine gland function from genetic or acquired causes can be restored by hormone replacement therapy. In general, steroid and thyroid hormones are replaced orally, whereas peptide hormones (e.g., insulin, GH) require injection. Gastrointestinal absorption and first-pass kinetics determine oral hormone dosage and availability. Physiologic replacement can achieve both appropriate hormone levels (e.g., thyroid) and approximate hormone secretory patterns (e.g., GnRH delivered intermittently via a pump). Hormones can also be used to treat diseases associated with glandular hyperfunction. Long-acting depot preparations of somatostatin analogues suppress GH hypersecretion in acromegaly or 5-hydroxyindoleacetic acid (5-HIAA) hypersecretion in carcinoid syndrome. Estrogen receptor antagonists (e.g., tamoxifen) are useful for some patients with breast cancer, and GnRH analogues may downregulate the gonadotropin axis and benefit patients with prostate cancer. Novel formulations of receptor-specific hormone ligands are now being clinically developed for more selective therapeutic targeting; examples include estrogen agonists/ antagonists and somatostatin receptor subtype ligands. Modes of hormone injection (e.g., for PTH) may also determine therapeutic specificity and efficacy. Improved hormone delivery systems, including computerized minipumps, intranasal sprays (e.g., for desmopressin [DDAVP]), pulmonary inhalations, and depot intramuscular injections, allow added patient compliance and ease of administration. Insulin delivered by inhalation has already been approved for use, and inhaled forms of GH and other hormones are under investigation. Cell-based therapies employing highly programmed pluripotent stem cells expressing a particular hormone or growth factor are also in development These approaches require novel administration systems to allow cell-derived endocrine products to reach their intended target.17 Novel technologies potentially marked prolongation in the half-life of peptide hormones, which would require less frequent administration. For example, a once-weekly preparation of exenatide, a GLP1 analogue used in the treatment of type 2 diabetes as a twice-daily injection, is undergoing clinical trials. Despite this tremendous progress, some therapies, such as insulin delivery to rigorously control blood sugar, still

require frequent administration by injection and close monitoring by the patient. Novel treatment systems that would link frequent monitoring of glucose levels to appropriate adjustments in the insulin dose promise to substantially reduce the burden of this disease. Hormones are biologically powerful molecules that exert therapeutic benefit and effectively replace pathologic deficits. They should not be prescribed without clearcut indications and should not be administered without careful evaluation by an appropriately qualified medical practitioner.

REFERENCES 1. Liu PT, Stenger S, Li H, et al. Toll-like receptor triggering of a vitamin D-mediated human antimicrobial response. Science. 2006;311:1770-1773. 2. Chawla A, Repa JJ, Evans RM, et al. Nuclear receptors and lipid physiology: opening the X-files. Science. 2001;294:1866-1870. 3. Mendel CM, Weisiger RA, Jones AL, et al. Thyroid hormone-binding proteins in plasma facilitate uniform distribution of thyroxine within tissues: a perfused rat liver study. Endocrinology. 1987;120: 1742-1749. 4. Mendel CM. The free hormone hypothesis: physiologically based mathematical model. Endocr Rev. 1989;10:232-274. 5. Safadi FF, Thornton P, Magiera H, et al. Osteopathy and resistance to vitamin D toxicity in mice null for vitamin D binding protein. J Clin Invest. 1999;103:239-251. 6. Palha JA, Fernandes R, de Escobar GM, et al. Transthyretin regulates thyroid hormone levels in the choroid plexus, but not in the brain parenchyma: study in a transthyretin-null mouse model. Endocrinology. 2000;141:3267-3272. 7. Palha JA, Episkopou V, Maeda S, et al. Thyroid hormone metabolism in a transthyretin-null mouse strain. J Biol Chem. 1994;269:3313533139. 8. Visser WE, Friesema EC, Jansen J, et al. Thyroid hormone transport by monocarboxylate transporters. Best Pract Res Clin Endocrinol Metab. 2007;21:223-236. 9. Hammes A, Andreassen TK, Spoelgen R, et al. Role of endocytosis in cellular uptake of sex steroids. Cell. 2005;122:751-762. 10. Rocheville M, Lange DC, Kumar U, et al. Receptors for dopamine and somatostatin: formation of hetero-oligomers with enhanced functional activity. Science. 2000;288:154-157. 11. Moore RY. Circadian rhythms: basic neurobiology and clinical applications. Ann Rev Med. 1997;48:253-266. 12. Aftab MA, Guzder R, Wallace AM, et al. Circadian and ultradian rhythm and leptin pulsatility in adult GH deficiency: effects of GH replacement. J Clin Endocrinol Metab. 2001;86:3499-3506. 13. Cauter EV, Leproult R, Plat L. Age-related changes in slow wave sleep and REM sleep and relationship with growth hormone and cortisol levels in healthy men. JAMA. 2000;284:861-868. 14. Melmed S. The immuno-neuroendocrine interface. J Clin Invest. 2001;108:1563-1566. 15. Huang SA, Tu HM, Harney JW, et al. Severe hypothyroidism caused by type 3 iodothyronine deiodinase (D3) in infantile hemangiomas. N Engl J Med. 2000;343:185-189. 16. Bianco AC, Salvatore D, Gereben B, et al. Biochemistry, cellular and molecular biology, and physiological roles of the iodothyronine selenodeiodinases. Endocrine Rev. 2002;23:38-89. 17. Graf T, Enver T. Forcing cells to change lineages. Nature. 2009; 462:587-594.

Clinical Endocrinology: A Retrospective,  13 The Clinical Approach to the Patient,  16 The Endocrinologist as Oncologist,  26 The Endocrinologist as Educator and Student,  28 Future Directions and Considerations,  28

CHAPTER CHAPTER 2  Clinical Endocrinology: A Personal View GILBERT H. DANIELS

CLINICAL ENDOCRINOLOGY: A RETROSPECTIVE I love practicing and teaching clinical endocrinology. As a young physician in the early 1970s, I was drawn to clinical endocrinology by its strong biochemical basis, elegant physiology, relative diagnostic clarity, and dramatic therapeutic efficacy. I liked the fact that clinical endocrinology made intellectual sense, demanded both clinical and laboratory expertise, and could also make patients feel better. With respect to biochemistry, I found that knowing the steroid hormone biosynthetic pathways was essential to understanding the adrenogenital syndrome. Knowing how catecholamines were metabolized made it possible to understand the diagnostic tests for pheochromocytomas. In other words, I realized that understanding the biochemical basis of an endocrine disorder could eventually lead to its proper diagnosis and treatment. Understanding the physiologic basis of an endocrine problem was equally critical. Dr. Fuller Albright was the first to describe hormone resistance and the ectopic

production of hormones. Albright’s physiologic insights also led him to develop the popular arrow drawings detailing the feedback loops between trophic hormones and end organs. Many diseases of hormone excess could be categorized using Albright’s reasoning. An exception is destructive thyroiditis, in which hyperthyroidism is caused by the uncoordinated release of stored thyroid hormone. This kind of hormonal excess does not occur in other endocrine organs. Manipulation of physiology allowed us to distinguish deficiency of growth hormone (GH) or cortisol (i.e., failure of levels to rise after insulin-induced hypoglycemia) from a nadir between pulses of these hormones. Conversely, failure to suppress cortisol after administration of the synthetic glucocorticoid dexamethasone (which does not register in the cortisol assay) was recognized to be a sign of “autonomous” cortisol production rather than a cortisol peak. Dr. Daniel Federman taught generations of endocrinologists to apply two simple rules with respect to pulsatile hormones—“If it’s low, stimulate it; if it’s high, suppress it”—to determine whether low or high hormone concentrations were physiologic or pathologic. 13

14   Clinical Endocrinology: A Personal View We found that fitting together all the pieces of an endocrine puzzle could lead to remarkably effective therapies. The administration of deficient hormones (GH for GH deficiency, thyroid hormone for hypothyroidism, cortisol for adrenal insufficiency, insulin for “juvenile” diabetes mellitus, pitressin tannate in oil for diabetes insipidus) was life-altering or even life-saving for patients, as well as immensely gratifying for clinicians. Eliminating excess hormone production produced equally beneficial and often dramatic results. Hyperthyroidism could be treated with medication, with targeted therapy using radioactive iodine, or, less commonly, with surgery. Surgery was required to treat the hormone excesses of Cushing’s syndrome, acromegaly, hyperparathyroidism, and pheochromocytoma. There were also significant limitations to our knowledge of clinical endocrinology in the early 1970s. Our ability to image the thyroid was restricted to radioisotope thyroid scans and the rare ultrasound study. It was almost impossible to image the pituitary or the adrenal glands, because computed tomography (CT scanning) and magnetic resonance imaging (MRI) were not yet available. Thyroid hormone excess was the only type of hormone excess that could be effectively treated nonsurgically. We had a limited knowledge of the spectrum and epidemiology of endocrine diseases (particularly mild functional disease and incidental structural disease) and their diagnostics and therapeutics. There were also endocrine diseases that had yet to be recognized or described, including endocrine disorders precipitated or caused by new therapies for nonendocrine disorders.

Modern Clinical Endocrinology In the last 40 years, our knowledge of endocrine physiology, pathophysiology, biology, molecular biology, and genetics has dramatically expanded. This new knowledge has changed many of the ways in which we diagnose and treat endocrine disorders. For example, greater understanding of the differential diagnosis and biologic mechanisms of hormone excess has made it possible to distinguish among clinically similar disorders and to identify new disorders. During the past 40 years, new research has helped explain previously puzzling clinical observations and also opened up entirely new areas of investigation and therapy. Some examples follow. • The hypokalemia of licorice administration and the hypokalemia of the ectopic adrenocorticotropic hormone (ACTH) syndrome caused by small cell carcinoma of the lung were understood only after the binding of cortisol to the mineralocorticoid receptor and its inactivation by the enzyme, 11β-hydroxysteroid dehydrogenase 2 was described. Inhibition of this enzyme by the licorice component, glycyrrhizic acid, was found to prevent inactivation of cortisol at the mineralocorticoid receptor; the dramatic overproduction of cortisol in the ectopic ACTH syndrome presumably overwhelms this enzyme. Cortisol becomes a potent mineralocorticoid receptor agonist in both situations.1 • Molecular biology was necessary to understand glucocorticoid-remediable hyperaldosteronism, in which a genetic crossover allows the ACTH-driven 11β-hydroxylase promoter to drive aldosterone synthesis instead of cortisol synthesis—a rare example of a hormone-overproducing syndrome without an increase in the number of cells making the hormone.

• Defining the role of 5α-reductase in testosterone metabolism led to the understanding of prepubertal “testosterone resistance” in individuals deficient in this enzyme, as well as potential therapies for prostate hypertrophy and androgenic hair loss. • Familial hypocalciuric hypercalcemia (FHH) was distinguished from primary hyperparathyroidism by means of an extremely low urine calcium excretion (and clearance). Once the calcium sensor was identified, FHH was proven to be caused by an inactivating mutation of that receptor. More recently, an astute endocrine fellow discovered that antibodies inhibiting the calcium sensor caused intermittent primary hyperparathyroidism in a patient with other autoimmune disorders.2 Conversely, mutations and anti­ bodies that activate the calcium sensor cause hypoparathyroidism. • New data have emerged about vitamin D. Vitamin D deficiency was identified as a common disorder in adults. The active form of vitamin D (1,25-dihydroxyvitamin D3, or calcitriol) was characterized as a hormone. It rapidly became an important therapy for hypoparathyroidism and for hereditary syndromes in which it is not produced. Increased production of calcitriol by active granulomas explained the hypercalcemia of sarcoidosis. • Recognition of hypophosphatemic syndromes such as tumoral osteomalacia was made possible by elucidation of the fibroblast growth factor (FGF23) pathways. • New therapies were developed for osteoporosis. The well-known industrial chemicals, bisphosphonates, were shown to be effective therapy; the well-described but little known anabolic bone effects of parathyroid hormone (PTH) were exploited in treating osteoporosis; and the recognition that different binders to estrogen receptors could sometimes mimic the action of estradiol and sometimes oppose it led to the use of selective estrogen receptor modulators (SERMs) for treatment of osteoporosis. • Parathyroid hormone−related protein (PTHrP) was found to be an important cause of the humoral hypercalcemia of malignancy and also to play a major role in bone biology. • The use of gonadotropin-releasing hormone analogues for the treatment of precocious puberty and for ovulation induction and as therapy for malignancies demonstrated the rewards of careful physiologic studies. • The observation that more insulin is released after oral glucose than after intravenous glucose administration led to the discovery of incretins as well as the clinical application of glucagon-like peptide-1 (GLP1) agonists for the treatment of diabetes mellitus. It is possible that incretins are responsible for the hypoglycemic syndromes that occur after gastric bypass surgery.3 Recently, labeled GLP1 agonists have been used to image insulinomas.4 • Blood sugar control was shown to be important to prevent the complications of diabetes mellitus. In addition, the diverse biochemical and physiologic bases of hormone resistance were elucidated, the spectrum of hormone resistance was expanded, and some of this new knowledge led to several important therapies for syndromes of hormone resistance. • Laron dwarfism is a form of GH resistance caused by a mutation in the GH receptor. Once insulin-like growth factor type 1 (IGF1), the downstream effector

Clinical Endocrinology: A Personal View    15

of GH, was identified and synthesized, growth could be induced by bypassing the missing step. • Pseudo–vitamin D deficiency rickets is caused by a deficient enzyme (25-hydroxyvitamin D3 1αhydroxylase) and can be treated by administration of calcitriol, the product of that enzyme. However, if the receptor for calcitriol is missing, then true “resistance” is present and cannot be directly overcome. • Pseudohypoparathyroidism is often caused by mutations in the Gs signaling protein and may be associated with more subtle forms of resistance to hormones other than PTH (e.g., resistance to thyroid-stimulating hormone [TSH]). In pseudohypoparathyroidism type 1A, the renal proximal tubule is resistant to the actions of PTH (because of maternal imprinting in the proximal tubule, where only one allele of the Gs gene is active), but the bones are not resistant. Therefore, resistance may vary among tissues. • Hormone resistance may be acquired. For example, antibodies to the insulin receptor cause diabetes mellitus with severe insulin resistance requiring thousands of units of insulin daily; obesity worsens the insulin resistance of type 2 diabetes; starvation causes GH resistance; and renal failure causes PTH resistance. • Hormone resistance at one receptor can result in spillover to other related receptors. For example, insulin resistance leading to elevated insulin can cause hirsutism by stimulating IGF receptors on the ovary. We now recognize many autoimmune endocrine disorders and are beginning to understand their physiology.

Changing Spectrum of Endocrine Diseases Most of the endocrine disorders recognized before 1970 were clinically severe and therefore easily diagnosed. In contrast, many of the endocrine disorders identified after 1970 have been clinically more subtle, easy to overlook, and far more prevalent. As a result, the percentage of severe endocrine disorders has declined over the years, whereas the percentage and prevalence of less severe endocrine disorders have risen sharply (Fig. 2-1). The following are some examples of less severe endocrine disorders that are now commonly diagnosed. • “Asymptomatic” hyperparathyroidism is often diagnosed through the routine measurement of serum calcium levels with autoanalyzers.

Disease severity at presentation Disease prevalence

• Mild thyroid disease (“subclinical” hyperthyroidism or hypothyroidism) can be detected and studied with the use of readily available sensitive TSH assays. • “Subclinical” Cushing’s syndrome caused by autonomous adrenal nodules can now be detected by appropriate diagnostic testing after the incidental discovery of an adrenal nodule, • Type 2 diabetes mellitus is now considered to be the most common cause of diabetic microvascular complications and a leading cause of cardiovascular morbidity and mortality. • Insulin resistance without diabetes mellitus is now considered to be a condition that poses a significant cardiovascular risk. • Polycystic ovarian disease, once thought to be a rare disease of obese, hirsute women with infertility, has been shown to be a common cause of irregular menses, hirsutism, and insulin resistance. • The widespread availability of bone densitometry has led to a greater understanding of the spectrum and population consequences of low bone mass. • Modern diagnostic algorithms have revealed that 8% to 10% of hypertensive patients have primary hyperaldosteronism. Primary aldosteronism may also be associated with cardiovascular morbidity independent of its effect on blood pressure.5 Our ability to diagnose ever-milder stages of endocrine disorders has brought with it a variety of challenging new questions for endocrinologists and epidemiologists: • What constitutes “normal” levels of hormones and metabolites? Are “normal” and “optimal” levels synonymous? Which of these two levels is more physiologically relevant? Is the optimal physiologic level also optimal for disease prevention? • Is treatment beneficial for patients with mild endocrine disorders? If intervention is beneficial, when should it be done? Does the treatment of mild endocrine and metabolic abnormalities ultimately benefit society? It is important to keep in mind that there is not yet a general consensus about what defines “normal,” “optimal,” and “abnormal” levels for any given hormone or metabolite and that these definitions are continuously undergoing revision to reflect the latest research. Some define “normal” in purely statistical terms. Others base their assessment of “optimal” levels on relevant physiologic or pathologic changes. For example, vitamin D deficiency was first recognized as a level of vitamin D so low as to cause rickets and osteomalacia. Investigators then modified the definition of normal to be the level of vitamin D below which PTH levels start to rise. More recently, some have proposed that the optimal level of vitamin D is the one that maximizes bone density in population studies and clinical trials. Diabetologists have used certain glucose concentrations associated with adverse outcomes to define diabetes mellitus, impaired glucose tolerance, and elevated fasting glucose concentrations. The problem is that there is no universally accepted standard for these values.

Radiology and Endocrine Epidemiology

1970

2010

Decades Figure 2-1  Changing pattern of disease severity over time.

In the 1960s, the great endocrinologist, Dr. George Thorn, reportedly explained in mock exasperation, “Thank goodness we can’t feel the adrenal glands!” He was referring to what at the time seemed like an overwhelming number of thyroidectomies being performed to treat palpable goiters. His fear was that once abnormalities became detectable in

16   Clinical Endocrinology: A Personal View other glands, the inevitable consequence would be an exponential increase in the number of patients requiring evaluation and treatment. Forty years later, we have come to realize how prescient Thorn was. Advances in radiology now allow us to scrutinize not only the adrenal gland but every other endocrine gland as well. The use of diagnostic ultrasound, CT scans, and MRIs of the head, neck, and abdomen has led to the clinical discovery of thyroid, pituitary, adrenal, and pancreatic tumors that were previously found only at autopsies or during surgery. The twin epidemics of these incidentalomas and the mild endocrine diseases discussed earlier has created a new dilemma: there are simply too many patients to be cared for by endocrinologists, even with the help of trained endocrine nurses and assistants. The result is precisely what Thorn had foreseen: all this new radiological information has led to the discovery of numerous “incidentalomas,” which in turn has led to a need for diagnostic evaluation and treatment. Part of the endocrinologist’s new job description, therefore, should be to educate primary care physicians and nurses in the optimal care of patients with these endocrine disorders.

“New” Endocrine Disorders One of the most exciting parts of being a clinical endocrinologist is that new or newly recognized endocrine disorders continue to be discovered, and clinicians are often the ones who find them. The story of one such discovery was often told by the legendary thyroidologist, Dr. Sydney Ingbar. A clinician called Ingbar to discuss a hyperthyroid patient who had a nil radioactive iodine uptake and a painless thyroid. Ingbar led the clinician through the standard differential diagnosis excluding iodine excess, exogenous thyroid hormone, struma ovarii, and painful (DeQuervain’s) thyroiditis. Ingbar eventually concluded that there was no such disease. In fact, the patient turned out to have what was subsequently named painless or silent subacute thyroiditis (painless destructive thyroiditis). Painless subacute thyroiditis has since been found to occur in 5% to 10% of all women in the postpartum period (postpartum thyroiditis). It had simply been unrecognized. For Ingbar, the story taught an important lesson: when making a diagnosis, the clinician must avoid a rush to judgment and remain open to the possibility of new diseases. Other endocrine diseases that have been recognized in recent years include the following: • Endocrine diseases caused by nonendocrine drugs. Many drugs (e.g., amiodarone, lithium, sunitinib) and immune modulators (e.g., denileukin diftitox, interleukin, and interferon in hepatitis C) also cause autoimmune thyroid disorders, and painless subacute thyroiditis in particular. A new B and T lymphocyte– depleting drug for multiple sclerosis (alumtuzemab) induces Graves’ disease in 12% of recipients. Etomidate, an anesthetic agent, may produce adrenal insufficiency. • Somatostatin- and glucagon-secreting tumors of the pancreas • Maturity-onset diabetes of the young (MODY), which results from a number of different specific biochemical abnormalities • Lymphocytic hypophysitis, an autoimmune cause of (postpartum) hypopituitarism, possibly related to the postpartum rebound of immune inhibition. This disorder may also be induced during cancer therapy with

antibodies targeting cytotoxic T-lymphocyte antigen 4 (CTLA4) • Human immunodeficiency virus (HIV) infection, the therapy for HIV, and the consequences of acquired immunodeficiency syndrome (AIDS) cause a spectrum of endocrine disorders, particularly ones involving the adrenal and the thyroid. Although endocrinology has changed fairly dramatically over the last 40 years, the clinical approach to the patient is based on a series of little-changed principles. The following sections present some insights and approaches that I have learned and taught over many years.

THE CLINICAL APPROACH TO THE PATIENT An endocrinologist should always view the patient’s condition as a whole rather than exclusively as an endocrine problem. Skillful questioning, personal interaction, careful deliberation, and sound clinical judgment are essential. The clinical endocrinologist has to fill many roles at once—radiologist, pharmacologist, physiologist, epidemiologist, public health physician, geneticist, oncologist, and educator. But perhaps the most important role is that of internist, because the endocrine system affects every system of the body. For example, caring for a patient with diabetes mellitus involves monitoring and treating not only blood sugar but also cholesterol levels, blood pressure, kidney function, vision, the cardiovascular system, and the nervous system. The process of clinical care begins with a carefully performed and recorded history and physical examination. During the history and physical examination, the endocrinologist must consider not only all the possible endocrine causes but also all the possible nonendocrine causes for the patient’s symptoms. It is unacceptable to dismiss a patient by saying, “This is not an endocrine problem,” without, if possible, suggesting alternative explanations for the symptoms. • If a patient has neck tightness and intermittent hoarseness, gastroesophageal reflux disease is a more likely etiology than goiter. • Severe fatigue (possibly in conjunction with hypogonadism, hypertension, and catecholamine excess) might be related to sleep apnea and may be suspected by asking about lack of restful sleep, the presence of snoring, and collar size (a collar size greater than 17 inches is often indicative of sleep apnea). Sleep apnea is a much more likely cause of fatigue than a minimally elevated serum TSH. • Depression, iron deficiency, and sleeplessness resulting from menopausal hot flashes should also be considered when a patient complains of fatigue. • A patient with persistent anxiety after treatment of Graves’ hyperthyroidism needs more than to be told that there is no endocrine basis for his or her anxiety because the thyroid function is now normal. Based on my observations, these symptoms may be a form of “programmed” anxiety resulting from long-standing hyperthyroidism. These patients often benefit from a clear explanation, cognitive behavioral therapy, and/ or anxiolytic therapy. • Always check the neck veins in a hyperthyroid patient with dyspnea to make sure that the dyspnea is caused by the hyperthyroidism per se rather than by congestive heart failure.

Clinical Endocrinology: A Personal View    17

• The “uncontrolled” hypertension of a patient with trivial elevation of urine metanephrines may be explained by a history of extensive use of nonsteroidal anti-inflammatory drugs6 or by the nonspecific metanephrine elevation of hypertension. • Weight loss with preserved appetite suggests hyperthyroidism, diabetes mellitus out of control, pheochromocytoma, malabsorption, and possibly anorexia nervosa; weight loss with a poor appetite has other, potentially more serious explanations. However, elderly hyperthyroid individuals may also have anorexia. • Severe joint pains in a patient with autoimmune thyroid disease may represent rheumatoid arthritis or other nonendocrine autoimmune disorders. • Vague abdominal pains, unexplained weight loss, or increased requirement for thyroid hormone may be caused by celiac disease, which is more common in patients with autoimmune thyroid disease. An obvious but often overlooked point is that the patient should be told exactly what the diagnosis means once the clinician makes it. It is important to answer the patient’s questions, provide reassurance, and allay any unnecessary fears. For example, many patients with hyperthyroidism and weight loss continue to worry until they hear the key words, “This is not cancer.”

History and Physical Examination What can we learn from the initial clinical examination? A careful history and physical examination are the first steps in establishing a diagnosis. All endocrine fellows learn the classic symptoms and signs of hormone excess and deficiency, but learning the characteristic tempo of each disease is equally important. For example, acute onset of Cushing’s symptoms or hirsutism suggests a malignancy, and hyperthyroid symptoms lasting longer than 4 months effectively exclude destructive thyroiditis. The following are several simple but very useful diagnostic tips to keep in mind during the initial clinical examination: • Regular menses with molimina is the best sign of a normally functioning hypothalamic-pituitary-ovarian axis and essentially precludes the need for hormonal testing for this axis. • Erectile dysfunction without loss of libido is rarely a symptom of testosterone deficiency. • Most patients with Cushing’s syndrome complain of difficulty falling asleep. • When a patient with a neck or thyroid mass complains of difficulty swallowing, the first question should be, “Is it difficult to swallow food or saliva?” and not “Which surgeon would you like to see?” Difficulty swallowing saliva is caused by repetitive swallowing when there is nothing to swallow (globus). If the swallowing difficulty occurs with food, then a barium swallow will help determine whether the dysphagia is related to the neck mass. • Diabetes insipidus is often characterized by the sudden onset of thirst for ice cold water in particular. • Severe male hypogonadism may be associated with hot flashes. • Sudden growth of a neck mass in a patient with a history of Hashimoto’s thyroiditis raises the specter of a thyroid lymphoma. • Severe muscle cramps are often caused by severe hypothyroidism, particularly when the onset is acute.

A careful medication history should include ascer­ tainment of prescription medications, over-the-counter medications, vitamins, and supplements, because both prescription and nonprescription medicines can influence endocrine function. Several targeted chemotherapy agents and immunomodulatory drugs may induce endocrine disorders, particularly hyperthyroidism and hypothyroidism. A history of megestrol acetate (Megace) administration, recent intra-articular glucocorticoid injections, or extensive topical glucocorticoid administration may provide the clue to otherwise unexplained suppression of the hypothalamic-pituitary-adrenal axis. High-dose glucocorticoid therapy may explain the slightly low serum TSH in a patient referred for “subclinical hyperthyroidism.” A host of drugs have been shown to increase the metabolism of thyroid hormone or cortisol, and knowledge of these drugs and their uses may explain otherwise unexplained worsening of treated or untreated hypothyroidism or Addison’s disease. Many drugs and supplements inhibit levothyroxine absorption, and it is important to ask the patient whether any of them are being taken. A hyperthyroid patient with a nil radioactive iodine uptake may be ingesting large amounts of (iodine-containing) kelp or over-thecounter thyroid supplements that contain active hormone. Many drugs are associated with hyperprolactinemia, especially atypical antipsychotics such as risperidone. The use of anticoagulants may be associated with adrenal hemorrhage. Smokers are at increased risk for severe Graves’ ophthalmopathy. A careful family history is necessary to help diagnose those important endocrine disorders that are genetic or familial. An entirely different approach to diagnosis and therapy is necessary when the patient being evaluated for hyperparathyroidism is found to have a parent or sibling with the same diagnosis. A family history of diabetes mellitus may be an important clue when dealing with atypical hypoglycemia. Surreptitious use of insulin or oral hypoglycemics must be excluded. Remember that insulin is also available without a prescription. Knowledge of the past medical history and concomitant illnesses may provide important clues to current endocrine problems. A history of head and neck irradiation in childhood necessitates a thyroid ultrasound examination and a higher degree of concern when thyroid nodules are discovered. It also places the patient at increased risk for primary hyperparathyroidism. Infants, children, and fetuses who were exposed to radiation during the power plant meltdown near Chernobyl in 1986 are now at risk for thyroid nodules and thyroid cancer. A history of anti-cardiolipin syndrome may be a clue to the presence of Addison’s disease caused by adrenal hemorrhage. A recent CT scan with iodinated contrast may explain the sudden onset of hyperthyroidism in a patient with a long-standing nodular thyroid. Many helpful clues can be obtained from a careful physical examination. Be sure to look at the patient’s skin and hair during the physical examination. • Early gray hair (1 gray hair before age 30) or vitiligo is a marker of autoimmune disease. A hypopigmented thyroidectomy scar suggests that the surgery was for autoimmune thyroid disease (Graves’ disease or Hashimoto’s thyroiditis). • ACTH-driven hyperpigmentation (including the buccal mucosa, extensor surfaces, nipples, and recent scars) may be a sign of Addison’s disease, Nelson’s syndrome (growth of a corticotroph adenoma after adrenalectomy for Cushing’s syndrome), or ectopic

18   Clinical Endocrinology: A Personal View Cushing’s syndrome resulting from small cell carcinoma of the lungs. Increased sun tanning may occur in hyperthyroidism (because of hypermetabolism of cortisol with resultant ACTH overproduction). Increased melanin also occurs in hemochromatosis, but in this case it is the iron pigment that stimulates melanin production. • Angiofibromas and collagenomas are characteristic of multiple endocrine neoplasia (MEN) type 1 and may turn a “simple” case of hyperparathyroidism into a search for this familial syndrome. • Acanthosis nigricans is common with many disorders of insulin resistance. • Café-au-lait spots may suggest McCune-Albright syndrome or neurofibromatosis type 1. • Skin tags may be a clue to the diagnosis of acromegaly (and may predict colonic neoplasms). Shaking hands with an acromegalic has often been likened to shaking hands with the Pillsbury doughboy. Although neck ultrasound has replaced thyroid examination in many offices, the value of the thyroid examination should not be underestimated. I believe that it is easiest to learn to feel the thyroid when facing the patient; the thyroid is palpated with finger pressure toward the trachea as the patient swallows. If a thyroid bruit is present, hyperthyroidism is almost invariably the result of Graves’ disease. If the thyroid remains large when hypothyroidism occurs after radioactive iodine treatment of Graves’ disease, the hypothyroidism is likely to be transient (“thyroid stunning”). Careful palpation will distinguish the tender thyroid of painful subacute thyroiditis from general neck tenderness or a painful nodule. Although it is often missed on ultrasound, tracheal deviation is an important physical finding of an asymmetric, possibly substernal thyroid mass. There are several other points to consider during the physical examination: • The presence of clitoromegaly directs the differential diagnosis of hirsutism toward more aggressive pathologies (hyperthecosis or androgen-secreting tumor). • Don’t forget to examine the testes, because they may provide the only clue to the diagnosis of Klinefelter’s syndrome. • Anosmia (e.g., inability to smell coffee) may help explain the cause of central hypogonadism (Kallmann’s syndrome). • Feel carefully for an abdominal mass, which will allow you to diagnose an adrenocortical carcinoma (ACC) in a patient with Cushing’s syndrome. • Fawn-like wrinkling around the eyes, failure of the hairline to recede, and a chubby habitus provide almost instant recognition of untreated adult male panhypopituitarism. • The presence of torus palatinus may be a marker for increased bone density caused by activating mutations of low-density lipoprotein receptor–related protein 5 (LRP5), although this finding is not specific. • Sudden diastolic hypertension may be caused by hypothyroidism, particularly if it occurs acutely after radioactive iodine therapy. Postural hypotension in a hypertensive patient may be found in patients with pheochromocytomas or primary aldosteronism with hypokalemia. Severe postural hypotension, often resulting from autonomic insufficiency, is an important cause of fatigue in the elderly. • Filling in of the supraclavicular fat pads may be an early sign of Cushing’s syndrome.

Laboratory Testing Endocrinologists are known among their medical colleagues as physicians who order blood and urine tests. Indeed, one of Dr. Fuller Albright’s guiding principles was to “measure something.” But it is always important to know what you are measuring, why you are measuring it, and what the advantages and pitfalls of the assay are. Testing for the sake of testing is both foolish and expensive. Be aware that assays can be misleading. In the 1970s, many malignancies were thought to secrete PTH because the assays used at the time were not specific. We now know that many malignancies produce PTHrP, but very few produce PTH. Although almost all patients with pheochromocytomas have elevated urine metanephrines and/or fractionated catecholamines, minimal elevations above normal limits have little specificity for pheochromocytoma. Learn the cutoffs that provide greater specificity. The 24-hour urine free cortisol level is often falsely elevated at high urine volumes (>4 L/day). Serum gastrin and chromogranin A are usually markedly elevated in patients taking proton pump inhibitors. Take time to learn about the assays that you use. When serum thyroxine (T4) was the major thyroid test, we had to learn the causes of falsely high and low T4 concentrations. More recently, we have learned that most modern free T4 assays give surprisingly low readings during pregnancy, whereas dialyzable free T4 is normal or high at that time. A total T4 measurement may be helpful in this situation. Even though TSH is a near-perfect thyroid function test in outpatients, the clinician must still have a clear understanding of its physiology and potential pitfalls. A normal serum TSH concentration essentially excludes hyperthyroidism and hypothyroidism if the hypothalamus and pituitary are normal. Use of algorithms that measure only TSH if it is normal but add free T4 if the TSH is high or triiodothyronine (T3) and free T4 if the TSH is low reduces the need for further testing and is more efficient and costeffective than ordering everything at once or reordering after an isolated abnormal TSH measurement. We also use an algorithm for patients taking levothyroxine that adds free T4 only if the TSH is less than 0.05 IU/L, because we have determined that information from the free T4 measurement is useful only when a low TSH is in this range. Almost all patients with a normal TSH and a slightly low free T4 are euthyroid. The low free T4 level is likely to be a laboratory aberration (and probably should not have been measured in the first place). Although free T3 measurements have become popular in diagnosing hyperthyroidism, I find that total T3 concentrations are sufficient. Many primary care physicians and some endocrinologists do not understand the exponential relationship between thyroid hormone and TSH and therefore cannot explain it to their patients. A 50% decrease in free T4 causes a 90-fold increase in TSH. Hence, a doubling of TSH from 5 to 10 mIU/L represents at most a decrease of a few percentage points in thyroid hormone concentration and not a 50% decrease. Once patients understand this simple relationship, their concerns about “wild swings” in their hormones can be allayed. It should be noted that TSH is less useful and can actually be misleading in patients with pituitary or hypothalamic disease and in acutely ill hospitalized patients. We must always interpret the results in the context of the patient’s clinical state. A low TSH (and a low free T4) may signify central hypothyroidism. A slightly high TSH (with

Clinical Endocrinology: A Personal View    19

a very low free T4) occurs in hypothalamic hypothyroidism in which the TSH is biologically inactive. Hyperthyroidism may occur with an elevated TSH (and a high free T4) if it is caused by a pituitary tumor, but a pituitary tumor also can cause hyperthyroidism with a normal TSH (due to heightened TSH bioactivity). It takes great clinical acumen to suspect hyperthyroidism and order additional thyroid function tests when the TSH is normal. Severe illness (“sick euthyroid”) may inhibit TSH release, causing a low serum TSH and, over time, a low serum free T4. TSH measurements can be misleading even with an intact hypothalamic-pituitary-thyroid axis in a healthy individual. I recently saw an elderly woman who had become progressively more hyperthyroid because her elevated TSH could not be brought down to normal with supraphysiologic doses of thyroid hormone prescribed by her personal physician. After a brain MRI to exclude a TSHsecreting pituitary tumor, she was sent for consultation to exclude thyroid hormone resistance despite being clinically hyperthyroid. We eventually discovered that she produced heterophilic antibodies that interfered with the TSH assay, resulting in falsely high TSH readings with some TSH assays. TSH measurements in a different laboratory confirmed an undetectable serum TSH until thyroid hormone was stopped and the TSH returned to normal. Heterophilic antibodies can interfere with other assays as well. Similar to the example just described, a very high prolactin concentration in a woman with regular menses might suggest macroprolactinemia, a false elevation of the serum prolactin level caused by hormone-binding gamma globulins. Some valuable insights may be gleaned from basic laboratory tests: • If the TSH remains low and stable over many years (e.g., 0.2 IU/L), nodular disease (toxic nodular goiter) is the likely cause. The mild hyperthyroidism of Graves’ disease often waxes and wanes over time. • Stable hypercalcemia that dates back years is very likely to be caused by hyperparathyroidism. • If the serum calcium concentration is consistently at the upper range of “normal,” it is likely to be abnormal. Although many laboratories consider serum calcium levels lower than 10.5 mg/dL to be normal, most individuals with consistent calcium measurements in the range of 10.2 to 10.5 mg/dL will prove to have an abnormality of calcium metabolism, usually primary hyperparathyroidism. • The puzzling rise and fall of serum calcium and PTH in patients with primary hyperparathyroidism is caused by the negative feedback of calcium on the abnormal parathyroid glands: the higher the serum calcium, the lower the PTH concentration. When hyperparathyroid patients take in additional calcium, the calcium may rise and the PTH may fall. If calcium intake is decreased or vitamin D deficiency is present, the serum calcium concentration is often lower (or even normal), and the PTH is higher. • A low serum phosphate level may be a clue to osteomalacia, vitamin D deficiency, hyperparathyroidism, or muscle weakness (phosphate depletion syndrome). However, a low serum phosphate concentration after a meal or during intravenous glucose administration is related to intracellular shifts in phosphate and does not require diagnostic evaluation. After thyroid or parathyroid surgery, a high-normal or elevated serum phosphate concentration suggests hypoparathyroidism. An elevated serum phosphate level may be also found in acromegaly.

• I always check the serum albumin level whenever I check the serum calcium, and I then correct for the albumin concentration. Hemoconcentration (related to use of the tourniquet during phlebotomy) may cause an elevated serum calcium due to an elevated serum albumin level. Hemodilution, often resulting from the administration of intravenous fluids, lowers the serum calcium and albumin concentrations. • Male hypogonadism is associated with a fall in the hematocrit to the normal female range. Men with appropriate signs and symptoms and a hematocrit in the range of 35% to 40% should be evaluated for hypogonadism. • An elevated alkaline phosphatase concentration is common in patients with untreated Graves’ disease. With treatment, the alkaline phosphatase level usu­ ally rises further (as bone heals) and may remain above normal for up to 1 year after the patient is euthyroid. • A white blood cell count with 1% or 2% eosinophils almost always excludes overt Cushing’s syndrome. • A low neutrophil count may be present in severe Graves’ disease before therapy (because of immune destruction of granulocytes by antibodies targeting granulocyte TSH receptors).7 This is not a contraindication to anti-thyroid drug therapy. • Hyponatremia without hyperkalemia may be a consequence of glucocorticoid deficiency from central causes. Learn the appropriate timing for tests and how to interpret their results: • A low morning cortisol level (18 µg/dL) usually excludes adrenal insufficiency. • Serum prolactin may rise after a meal and therefore should be measured in the fasting state to determine whether minor elevations are abnormal or related to food. • Although the release of testosterone is pulsatile, testosterone has a diurnal variation, with concentrations being higher in the morning and lower in the evening. As it has been said, only half-jokingly: “If you want to study normal (eugonadal) men, draw a testosterone level in the a.m. If you want to study hypogonadal men, draw a testosterone level in the afternoon.” Many patients have been unnecessarily treated with testosterone based on a single afternoon testosterone concentration below the normal range. Be aware that many so-called free testosterone assays report low values even in eugonadal men. • Once suppressed, the serum TSH may remain undetectable for months, even after the patient is euthyroid. Similarly, the hypothalamic-pituitary-adrenal axis may remain suppressed for 1 year or longer after successful treatment of Cushing’s syndrome or weaning from glucocorticoid excess. Take advantage of clinical serendipity: • The most useful time to measure serum thyroglobulin in patients with well-differentiated thyroid cancer is when the serum TSH is elevated (after injection of recombinant human TSH or withdrawal of thyroid hormone). If a patient with thyroid cancer is noted to be undertreated with a spontaneously elevated TSH, take advantage of that undertreatment by measuring serum thyroglobulin before increasing the levothyroxine dosage.

20   Clinical Endocrinology: A Personal View • When spontaneous hypoglycemia is reported, try to retrieve the blood sample for an insulin measurement. • A fully suppressed serum TSH in a patient taking modest doses of levothyroxine suggests thyroid autonomy; a thyroid scan performed when the TSH is low can confirm this suspicion. Note that it is better to perform a “suppression scan” in a patient with a thyroid nodule while the patient is taking thyroid hormone than to stop thyroid hormone before the scan. Think physiologically: • A mid-normal or high-normal PTH level in the face of hypercalcemia is inappropriate and hence diagnostic of primary hyperparathyroidism. Similarly, a low PTH level with hypercalcemia excludes hyperparathyroidism. An astute clinical investigator evaluated a critically ill patient with severe hypercalcemia, a low PTH, and a markedly elevated 25-hydroxyvitamin D concentration. Using physiologic reasoning, he concluded that exogenous vitamin D had to be the cause and eventually tracked the source to a miscalculation of the dose of vitamin D added to milk at a specific dairy.8 • A “normal” level of follicle-stimulating hormone (FSH) in a postmenopausal woman is inappropriately low and probably indicates a pituitary or hypothalamic problem. Conversely, an elevated postmenopausal FSH is strong (but not conclusive) evidence in favor of normal pituitary function, because loss of gonadotropin function is often an early sign of pituitary failure. Remember that starvation or acute illness can temporarily suppress the serum FSH even when the axis is intact. • Low levels of plasma cortisol and ACTH in a patient with clinical Cushing’s syndrome suggest the use of a potent glucocorticoid (e.g., dexamethasone) that is not measured in the cortisol assay. An alternative explanation is recent administration and withdrawal of supraphysiologic glucocorticoid. The ability to use the appropriate minimum number of laboratory tests is a hallmark of a thoughtful clinical endocrinologist. Efficient use of tests to exclude endocrine diagnoses is particularly important. A normal serum TSH excludes most cases of hyperthyroidism or hypothyroidism. A normal plasma concentration of metanephrines excludes most cases of pheochromocytoma. A normal midnight salivary cortisol or low-dose overnight dexamethasone suppression test effectively excludes most instances of Cushing’s syndrome. A normal aldosterone-to-renin ratio effectively excludes primary hyperaldosteronism. An elevated morning urine osmolality argues against diabetes insipidus. A low GH level after glucose administration or a normal basal IGF1 level effectively excludes acromegaly. Unfortunately, there is a tendency among some endocrinologists to “check off all the boxes” when ordering tests. Remember, Dr. Albright said to “measure something” not to “measure everything.” I see many patients in consultation who have undergone every possible thyroid or adrenal test, sometimes multiple times, even though most of these tests were not needed to make the diagnosis. Take the time to think carefully before ordering laboratory tests and to evaluate the diagnostic value and expense of each test. • At our institution, the charge for a corticotropinreleasing hormone stimulation test (with eight ACTH measurements) is approximately $3500. A clinician

should make sure that expensive tests like this one are absolutely necessary before ordering them. • It is extremely important to measure thyrotropin receptor antibodies (TRAb) in pregnant patients previously treated with radioactive iodine or surgery for Graves’ disease. High-titer TRAb may predict fetal or neonatal Graves’ disease. Institution of appropriate therapy in a timely fashion can be life-saving or lifealtering for the newborn. Although many endocrinologists routinely (and often unnecessarily) measure TRAb serially in nonpregnant patients with known Graves’ disease, they often neglect to measure these antibodies in pregnant women previously “cured” of Graves’ disease. • Although it is relatively inexpensive, a 24-hour urine test for 5-hydroxyindoleacetic acid (5-HIAA) rarely has a true-positive result and is often ordered unnecessarily; most true-positive findings are in patients in whom the diagnosis of carcinoid syndrome can be made clinically. • Sometimes common sense is superior to standard testing algorithms. If you suspect primary adrenal insufficiency in a hyperpigmented patient, measure the serum cortisol and plasma ACTH. A high ACTH with a low cortisol level is diagnostic of primary adrenal insufficiency. Guidelines are a guide, not gospel. The role and benefit of screening tests is problematic. Bear in mind that although some screening tests are generally accepted, the value of others remains controversial. Screening for gestational diabetes is recommended based on data suggesting improved outcomes, but even here the benefits are debated. The value of TSH screening in pregnant women or in the general population is hotly debated. Many European and some North American endocrinologists recommend measurement of serum calcitonin in all patients with thyroid nodules, despite the fact that the relative value of this determination is unknown in terms of benefit (discovery of small medullary thyroid carcinomas) and harm (unnecessary surgery for false-positive calcitonin results or possibly innocent medullary microcarcinomas). In general, the presence of strong feelings on both sides usually indicates that the data suggesting benefit are weak or absent. Although each clinician will decide whether to screen for specific diseases in his or her own practice, major public health decisions about screening need to be supported by compelling data.

Radiology and Interventional Radiology I was taught to make an endocrine diagnosis before recommending imaging, and adhering to this sequence seems the best way to avoid ordering needless and costly radiologic scans. Imaging of the pituitary in a patient with tests diagnostic of adrenal Cushing’s syndrome is unnecessary. Patients who clearly do not have a pheochromocytoma do not need CT scans, MRIs, or 131I-meta-iodobenzylguanidine (MIBG) nuclear scans. On the other hand, patients referred for incidentalomas do require hormonal evaluation. Almost all adrenal nodules are benign. Major radiologic advances with CT and MRI imaging have allowed us to characterize most of these tumors as benign adenomas. Patients with benign adrenal tumors generally require endocrine evaluation for Cushing’s syndrome, subclinical Cushing’s syndrome, or pheochromocytoma. If the patient is hypertensive, primary aldosteronism should be excluded. Some of these patients with benign adrenal tumors are appropriately referred to surgeons based solely on tumor size.

Clinical Endocrinology: A Personal View    21

There is still uncertainty about the adrenal lesions that are characterized radiologically as “not simple adenomas.” Most of these lesions turn out to be benign, but all of them require careful follow-up, and some may require surgery. There is an unfortunate misconception among many endocrinologists that needle biopsy can distinguish a benign from a malignant adrenal cortical tumor. In fact, a needle biopsy can diagnose metastasis to the adrenals but only rarely distinguishes adrenocortical adenoma from ACCs. Use of a needle biopsy to diagnose ACC is therefore worthless, not to mention potentially harmful. Most adrenal cancers are large and readily diagnosed by CT or MRI. Given the clinical epidemic of thyroid nodules, many clinical endocrinologists use ultrasonography to interpret and biopsy thyroid nodules that are incidentally discovered during radiologic imaging for other conditions. Ultrasound can provide guidance about which thyroid nodules need to be biopsied. If multiple nodules are present, newer ultrasound algorithms should be used to help determine which nodules are suspicious, thereby avoiding the need to biopsy all the nodules. At the same time, keep in mind that simply because an ultrasound machine and a biopsy needle are available does not mean that they have to be used. Always consider the value of the diagnostic information that a biopsy could provide, particularly with ever-smaller thyroid nodules. For example, think about whether there really is an advantage to diagnosing a 5-mm papillary thyroid carcinoma. Likewise, common sense and judgment should lead to treating many nodules more conservatively in the aged. Whereas larger nodules in older patients may be aggressive malignancies, it is reasonable to tell octogenarians who present with five or six thyroid nodules of modest size that even if the risk of malignancy is about 10%, the risk of death from such malignancies is quite remote. These older patients can then decide for themselves whether to undergo one or more biopsies. Choosing not to have a biopsy seems particularly appropriate in this age group, especially when other potentially life-threatening illnesses are present. When consulted for a case of large multinodular thyroids, most endocrinologists check thyroid function and biopsy all concerning nodules. But in focusing on the individual nodules, they sometimes “miss the forest for the trees” and overlook the fact that the goiter is compressing the trachea or the esophagus. An important part of evaluating large goiters is CT imaging (without contrast) or MRI to exclude compression of local structures. Until recently, it was standard practice to obtain imaging studies of patients with primary hyperparathyroidism only if they were going to have surgery. But now clinicians sometimes image patients with “asymptomatic” primary hyperparathyroidism to help both the patient and the clinician decide whether surgery may be preferable to observation. For example, the ability to localize an abnormal parathyroid gland by ultrasound and/or sestamibi scanning might lead a clinician to recommend minimally invasive surgery for indications (e.g., unexplained fatigue) that do not meet the guideline criteria or to prevent bone loss in the future. Likewise, some patients will elect surgery rather than observation if they know that a minimally invasive procedure is feasible. But remember that parathyroid imaging should never be performed to establish the diagnosis of primary hyperparathyroidism; that task is solely in the domain of the testing laboratory. Endocrinologists need to be both mindful of physiology when interpreting radiographic images and judicious in their use of radiology. Some examples may be helpful.

• Pituitary macroadenomas with minimal prolactin elevation should be treated as nonfunctioning tumors rather than prolactinomas. Dopamine agonists are usually ineffective in patients with large tumors and minimal prolactin elevations. The minimal prolactin elevation in pituitary macroadenomas is usually caused by either disinhibition of prolactin production or a tumor that produces prolactin but inefficiently. • A sellar mass in a patient with diabetes insipidus is unlikely to be a simple pituitary adenoma; it is more likely to be a craniopharyngioma or the result of infiltrative disease. • Diffuse enlargement of the pituitary after pregnancy is most likely caused by lymphocytic hypophysitis. • Primary hypothyroidism can cause diffuse reversible pituitary enlargement and mild hyperprolactinemia. Serum TSH should therefore be measured in cases of diffuse pituitary enlargement and as part of an evaluation for hyperprolactinemia. • A 24-hour radioactive iodine uptake of 5% (normal, 10% to 30%) in the face of an undetectable TSH is characteristic of thyroid autonomy rather than destructive thyroiditis (in which the uptake must be nil). A reason should be sought for low radioiodine uptake (e.g., recent iodine exposure). • Given the growing concern about possible side effects of radiation from CT imaging, careful thought should be given to the use of repeat CT imaging (e.g., for adrenal nodules). In certain cases, invasive radiologic procedures are necessary before surgery to localize the specific tumor causing the endocrine hyperfunction. Because incidentalomas are common, a pituitary adenoma seen on MRI in a patient with pituitary Cushing’s syndrome or an adrenal nodule seen on CT scanning in a patient with primary aldosteronism cannot be assumed to be the site of hormone overproduction. Even in the presence of MRI- or CT-documented pituitary or adrenal adenomas, pituitary corticotroph adenomas often require localization with bilateral inferior petrosal sinus catheterization; aldosteronomas often re­ quire localization or exclusion with bilateral adrenal vein catheterization; and some insulinomas require intra-arterial calcium infusion with venous catheterization. All of these invasive radiologic procedures should be performed at specialized centers with extensive experience and wellestablished expertise.

Genetic Testing and Family Screening Endocrinologists are increasingly called upon to discuss genetic testing with their patients. Although it may seem like a straightforward matter of checking a box or ordering a blood test, the clinician should keep in mind that genetic testing has the potential to create profound psychological and practical challenges for patients and their families. We therefore recommend genetic counseling before testing, so that patients are prepared for the various problems they may subsequently encounter. For example, before genetic testing that might possibly identify a family member as a gene carrier for a serious condition, a genetic counselor may recommend the purchase of life insurance, because insurance could be extremely difficult to purchase if the patient is found to have the abnormal gene. Practical application of clinical tumor genetics began before the advent of gene testing with studies of familial medullary thyroid carcinoma (MEN2). The recognition of this dominantly inherited malignancy led to information about the natural history of the disease, rules for family

22   Clinical Endocrinology: A Personal View screening, stimulation tests (pentagastrin and calcium) to diagnose early disease, prophylactic surgery at the first sign of abnormality, and clinical observations that allowed one to predict the risk of developing the disease over time even if the screening test remained normal. The recognition of mutations in the RET proto-oncogene made it possible to predict the risk of inheriting MEN2 with greater precision and nuance, and genetic testing replaced endocrine testing as the standard of care for diagnosis of asymptomatic carriers of MEN2. For the first time, individuals in families with known mutations could be excluded as carriers of the disease if their RET test was normal. Conversely, carriers of the abnormal gene could be discovered at an early age. As genotype-phenotype correlations improved, recommendations about the appropriate time for testing and prophylactic surgery were modified. When caring for patients with pheochromocytomas, it is important to realize that a significant minority of these tumors (up to 25%) have a germline mutation that is responsible for the tumor. In fact, the first patient ever diagnosed with pheochromocytoma recently had her pedigree traced to an MEN2 kindred.9 Although these tumors may be familial, if the patient presents with a de novo mutation, the family history will not be revealing. When a patient with a pheochromocytoma is young, has bilateral disease, or has an extra-adrenal tumor, genetic testing should be strongly considered. Such genetic testing should look for mutations in the von Hippel-Lindau tumor suppressor gene (VHL), the gene encoding succinate dehydrogenase complex subunit B (SDHB) and also SDHD and SDHC (newly recognized abnormalities), RET, and, less commonly, neurofibromin 1 (NF1). Mutations in SDHB may predict malignant behavior. Genetic testing may also be helpful in other circumstances: • Patients with the cribriform morular variant of papillary thyroid carcinoma often harbor the familial adenomatosis polypi gene (FAP). Discovery of this gene may prevent death from colonic cancer when it leads to performance of screening colonoscopies. • Patients with Cowden’s syndrome may have follicular thyroid carcinoma as well as breast cancer. We consider screening younger patients with follicular thyroid carcinoma for Cowden’s syndrome, particularly those who have a family history of breast cancer. • Parathyroid carcinoma is a rare disease that may have a genetic predisposition, particularly when it is associated with jaw tumors and abnormalities of the other parathyroid glands. But a surprisingly high number of patients with apparently sporadic parathyroid cancer harbor familial mutations in CDC73, the cell division cycle 73 (parafibromin) gene (formerly called hyperparathyroidism-jaw tumor syndrome 2, or HRPT2)—an observation that suggests that genetic testing may be considered for all such patients. • When patients with hyperparathyroidism are found to have four-gland hyperplasia, we often suggest that other family members be screened for hyperparathyroidism. • We recommend family screening for individuals with FHH to help family members avoid unnecessary surgery. • Patients with Zollinger-Ellison syndrome and young patients with hyperparathyroidism or islet cell tumors should be evaluated for possible MEN1. This is also true for patients who have evidence for two possible MEN1 tumors. A careful family history is paramount.

Although gene testing is available for MEN1, mutations may be missed. Even if a mutation is present, careful screening is preferable to prophylactic surgery. With or without genetic testing, both family screening and continued surveillance are important.

Therapeutics Do all endocrine abnormalities require therapy? According to Sir William Osler, “A desire to take medicine is, perhaps, the great feature which distinguishes man from other animals.”10 To paraphrase Osler, one could say that what distinguishes endocrinologists from other physicians is their desire to replace missing hormones and to take away excess hormones. But as we shall see, logic, a desire to help, and abnormal laboratory tests do not necessarily lead to appropriate therapy. For many years, estrogen replacement therapy for postmenopausal women seemed to be both therapeutically logical and beneficial: it reduced hot flashes, increased the sense of well-being, and provided a cardiovascular benefit as demonstrated in cross-sectional epidemiologic studies. But appropriately controlled trials not only failed to confirm the cardiovascular benefit but demonstrated that this therapy could actually harm the patient. Additional studies are changing our ideas about postmenopausal hormone replacement therapy still further. Subclinical hypothyroidism (elevated TSH with a normal free T4) occurs in up to 20% of the elderly population. It is easily corrected with levothyroxine therapy leading to TSH normalization. But cross-sectional studies of the very elderly (85 years and older) suggest a survival advantage for untreated individuals with subclinical hypothyroidism.11 Furthermore, a significant minority of patients with subclinical hypothyroidism who are treated with levothyroxine develop subclinical hyperthyroidism, with its potential but unquantified consequences. Although subclinical hypothyroidism has been implicated in adverse pregnancy outcomes, there are still no controlled trials demonstrating improved outcomes with levothyroxine therapy. Despite this lack of evidence, many excellent endocrinologists demand screening for subclinical hypothyroidism, particularly for women contemplating pregnancy. My own feeling is that screening and therapy for subclinical hypothyroidism (or other mild endocrine disorders) may be reasonable for individual patients, but a recommendation that it become national policy is unwarranted in the absence of appropriate therapeutic trials. The increasing recognition of subclinical Cushing’s syndrome is a consequence of the radiologic detection of adrenal incidentalomas. A diagnosis of subclinical Cushing’s syndrome in a patient with an adrenal adenoma is most commonly based on the failure to suppress cortisol after overnight administration of dexamethasone, but the actual criteria for diagnosis vary considerably. There is no consensus about which dose of dexamethasone to use, which dexamethasone-suppressed cortisol value is diagnostic, or, more importantly, when surgical removal of the adrenal is advisable. Some patients clearly benefit from surgery, but the vexing issue for the clinician is that there are no definitive guidelines for recommending surgery.

Endocrine Hormone Replacement Therapy Endocrine replacement therapy has long been considered to be near-perfect therapy. The dramatic response of the patient with myxedema to sautéed sheep thyroid in the 19th century, the life-saving efficacy of insulin for diabetic

Clinical Endocrinology: A Personal View    23

ketoacidosis, the use of cortisol for Addison’s disease, and HRT for menopausal symptoms are some of the best examples. But not all endocrine replacement therapy is perfect or even nearly perfect. Indeed, many patients treated with various hormone replacements have decreased quality of life or shortened life expectancy compared with appropriate controls. • Finger-stick glucose testing, hemoglobin A1c measurements, and a recognition that control of blood sugar is critical have all helped revolutionize diabetes care. Equally important has been the development of “designer” short- and long-acting insulins. But the fact remains that current therapies for type 1 and type 2 diabetes still do not mimic endogenous glucose control. Although excellent glucose control has reduced microvascular complications significantly, only near-perfect glucose control early in the disease appears to prevent macrovascular complications. More rigorous control shows a lack of benefit or even potential harm late in the course of type 2 diabetes or after cardiovascular disease has developed.12 Hypoglycemia also continues to be a significant problem associated with insulin therapy. Looking ahead, if type 1 diabetes mellitus cannot be prevented, safer and readily available islet cell transplants may be the answer after the immunologic rejection problems are solved. For type 1 and type 2 diabetes mellitus, use of closed-loop glucose monitoring and insulin delivery systems early in the disease, if perfected, may provide near-perfect glucose control. Drugs designed to enhance islet cell regeneration may also turn out to be beneficial. • Epidemiologic studies suggest that hypothyroid patients receiving replacement therapy do not feel as well as matched controls. Many explanations have been offered. Undertreatment is one possibility, yet the difference remains even when serum TSH is normalized. Whether the lack of well-being reflects inadequate therapy or underlying unrelated symptoms is still unclear. The combination of liothyronine plus levothyroxine was initially hailed as a panacea for symptomatic, treated hypothyroid patients, but it was then rejected. Recent preliminary studies have suggested that only certain individuals (e.g., those with specific type 2 deiodinase polymorphisms) respond favorably to this combination therapy.13 This possibility raises the general question of pharmacogenetics, wherein patients have variable responses to treatment and require individualized therapies. • Therapy for Addison’s disease is certainly life-saving, but many studies have documented a less favorable quality of life and a possibly decreased life expectancy among treated individuals. It is still difficult to mimic the early-morning cortisol rise with our current therapeutics, but perhaps the newer, slow-release cortisol preparations given at midnight will prove effective. Most adrenal experts favor therapy with hydrocortisone, but I have found that many patients feel better with intermediate-acting glucocorticoids such as prednisone. • Unfortunately, there is no simple way to monitor glucocorticoid therapy. To prevent excess glucocorticoid administration, we use relatively crude indices such as blood pressure and presence or absence of pigmentation, normal electrolyte levels, and clinical signs of Cushing’s syndrome. Multiple daily measurements of cortisol in patients receiving hydrocortisone are used by some investigators but are quite cumbersome.

• The ability to measure bone density easily and reliably ushered in the modern era of diagnosis and treatment of osteoporosis. We continue to debate the appro­ priate duration and use of therapeutic agents for this condition, particularly when it is apparently mild. The bisphosphonates and other antiresorptives provide an excellent initial therapy for many patients but are far from being a panacea. PTH is anabolic for bone but has minimal effect on cortical bone mass. We await the development of newer safe agents with potent anabolic properties, particularly for cortical bone. • Currently available therapy for hypoparathyroidism in children often results in hypercalcemia or hypocalcemia. The inconvenience of administering the possibly more effective parenteral PTH will have to be weighed against the benefits, risks, and ease of current oral therapy with calcium and calcitriol. • Treatment of the adrenogenital syndrome in children must navigate between the Scylla and Charybdis of adrenal insufficiency and glucocorticoid excess. It is debatable whether endoscopic adrenalectomy plus adrenal replacement therapy is a preferable approach. • Topical testosterone gels provide serum testosterone concentrations that closely mimic mean endogenous testosterone levels. A goal for the future is to find a simple oral treatment that can mimic this pattern. There are no approved testosterone therapies for sexual dysfunction in women, largely because the efficacy of such therapies continues to be debated. • The use of GH purified from human pituitaries was abandoned after it was associated with the development of Creutzfeldt-Jakob disease. Recombinant human GH has since become the treatment of choice. Guidelines have been established for the diagnosis and treatment of GH deficiency caused by hypothalamic or pituitary disease in children and in adults. But GH deficiency in adults without structural pituitary or hypothalamic disease poses a challenge for clinicians in that they must weigh the uncertain longterm risks and benefits of GH therapy against the high cost of the drug.

Practical Considerations Several simple points I have learned over the years can make endocrine replacement therapy significantly easier for the patient: • Levothyroxine has a 7-day half-life. Patients who miss a pill on one day should be instructed to take two on the next. Up to seven pills once weekly may be prescribed. This flexibility often improves overall compliance with levothyroxine therapy. Be aware that some pharmacy handouts incorrectly inform patients not to make up missed doses. The same logic suggests that occasional missed bisphosphonate doses can be made up with mid-week doses (for drugs administered weekly). • Do not prescribe two different levothyroxine dosages for the same patient. For example, instead of prescribing 0.2 + 0.025 mg of levothyroxine, prescribe two 0.112-mg pills. Prescribing two different doses confuses the patient and also is more expensive because it requires two separate copayments for patients with insurance coverage. • To decrease or increase the dosage of levothyroxine by 15%, patients may, respectively, skip their medicine on one day per week or add one extra pill per week (e.g., on Sundays). When you are changing to a

24   Clinical Endocrinology: A Personal View new dosage, old prescriptions rarely need to be discarded. • Given the many interactions between levothyroxine and foods, vitamins, and supplements, patients should be given the option of taking their levothyroxine at night before bedtime. This will not interfere with sleep and may even provide smoother TSH control. • The treatment of chronic hypoparathyroidism includes calcium and calcitriol. I prefer to limit the dose of calcium to 500 to 600 mg three or four times daily. I adjust the calcitriol up or down to keep calcium within normal limits and avoid hypercalciuria. Some clinicians add hydrochlorothiazide to prevent hypercalciuria. Chlorthalidone is also effective. Hypokalemia caused by chlorthalidone is a common problem, perhaps because of its long duration of action. • Acute hypoparathyroidism may require higher initial calcium therapy. Calcium should be administered with meals to lower the high phosphate, which often prolongs the hypocalcemia. It is also advisable to avoid dairy products because of their high phosphate content. • Patients with Addison’s disease have certain special needs: they should always wear a necklace or bracelet informing others about their condition (e.g., Medical Alert); they need to be aware that their glucocorticoid dosage will have to be increased in times of stress or illness; and they or their family members should learn how to administer parenteral hydrocortisone sodium succinate in case of an emergency. Those patients who are taking daily hydrocortisone should consider keeping a supply of prednisone to use for extra doses when sick; this is particularly true for older patients, because they are more likely to be intolerant of fluid retention caused by excess hydrocortisone.

In-Patient Therapeutics Endocrine therapies in hospitalized patients require special attention. • After years in which a rather casual approach was taken to glucose control in the hospital, controlled trials in intensive care units (ICUs) showed that intensive glucose control increased survival in some ICUs. However, these conclusions have recently been challenged. • Nurses often become anxious when a non-ICU patient has an extremely high glucose concentration. However, they are quickly reassured once they learn that the situation can easily be brought under control with an intravenous insulin infusion that decreases the glucose by 80 to 100 mg/dL per hour. • The parathyroid glands may be damaged or removed, either inadvertently during thyroid or parathyroid surgery or intentionally during en bloc surgery for invasive head and neck cancers. The resulting acute hypocalcemia is often difficult to treat properly. Oral calcitriol takes time to work and cannot be administered to patients who are NPO. A simple temporary regimen to maintain normal serum calcium levels in this setting is the constant infusion of calcium. Four or five ampules of calcium gluconate (400 to 500 mg of elemental calcium) in 1000 mL of fluid is infused at a rate of 40 mL/hour. This regimen avoids the frequent peaks and troughs of bolus calcium administration. • Endocrinologists are often asked to consult on cases of hyponatremia. Appropriate clinical assessment of salt and water balance in these patients is the key to

appropriate diagnosis and therapy. Parenteral antidiuretic hormone (ADH) antagonists for the treatment of euvolemic hyponatremia are now available, but it remains unclear how and when these agents should be used. • Critically ill patients often develop abnormalities of thyroid function, including low T3, low T4 and free T4, and low TSH. These thyroid changes suggest central hypothyroidism. Because it is unclear whether these changes are adaptive or harmful, it is also unclear whether replacement therapy is appropriate for these patients. In the “euthyroid sick” syndrome, the serum TSH is rarely undetectable unless glucocorticoids or dopamine is administered. • There are new guidelines for treatment of cortico­ steroid insufficiency in acutely ill patients, based on responses to the ACTH stimulation test. Although these tests may define a population with glucocorticoidresponsive disease, it is difficult for me to categorize patients with very high serum free cortisol concentrations as adrenally insufficient.14 Whether they have a form of glucocorticoid resistance or another glucocorticoid-responsive illness is unknown. • Elderly patients with severe hypercalcemia or hyponatremia may be slow to recover their mental function. Do not be discouraged if the mental state remains abnormal for 2 to 7 days after the metabolic abnormality has been corrected.

Treatment of Hormone Excess As clinicians trained in internal medicine, endocrinologists would like to be able to treat all hormonal overproduction, including tumor-related hormone excess, with medication rather than surgery. The ideal therapy would be a medication given for a limited period that permanently eliminates the tumor as well as the hormone excess. Although medical therapy rarely approaches this ideal, it is often successful in controlling hormone excess. Graves’ hyperthyroidism is the most common condition of hormone excess treated by endocrinologists. Several important clinical points are worthy of consideration. In the 1930s and 1940s, surgery for Graves’ hyperthyroidism was extremely dangerous, and there was no safe way to prepare patients for surgery. As a result, the operative and perioperative mortality rates were high. After radioactive iodine and antithyroid drugs became available in the late 1940s, they rapidly became the treatment of choice. One problem when treating hyperthyroidism is that no regimen has been able to solve the dilemma of rebound weight gain, often to above baseline. Radioactive iodine has the advantage of treating hyperthyroidism permanently, but it also has certain disadvantages: it usually causes permanent hypothyroidism; it may cause worsening eye disease; it may cause hyperparathyroidism decades later; and, although there is no conclusive evidence, concern has been raised that the radiation may have additional harmful side effects. In selected patients, the addition of potassium iodide after radioactive iodine therapy for Graves’ disease accelerates the return to euthyroidism. Many young patients currently choose anti-thyroid drugs for the initial treatment of Graves’ hyperthyroidism. After years of debate, there is now a consensus that methimazole is the anti-thyroid drug of choice, as opposed to propylthiouracil, because of its higher potency, longer duration of action, and lower toxicity. Propylthiouracil is preferred in cases of thyroid storm because of its ability to

Clinical Endocrinology: A Personal View    25

inhibit the conversion of T4 to T3. It is also preferred in early pregnancy and when the patient has a minor allergy to methimazole. Patients must always be informed of the low but significant risk of fatal hepatic necrosis whenever propylthiouracil is prescribed. The 1 in 200 to 1 in 500 risk of agranulocytosis may be somewhat less with lower doses of methimazole. Hyperthyroid patients who decline both surgery and radioactive iodine therapy are usually given a therapeutic dose of methimazole, which is often tapered over time. There are several other beneficial regimens for treating hyperthyroid patients with methimazole, which endocrinologists should also consider: • Some patients treated with anti-thyroid drugs develop extremely unpleasant yo-yo swings in thyroid function. They remain hyperthyroid on too little medication, become hypothyroid on too much, and cycle between these two extremes. A block-replace regimen employing a full therapeutic dose of methimazole and the addition of levothyroxine allows stable thyroid function. This approach may also be useful for students going away to school and for patients far removed from laboratories where blood tests can be monitored. • There are some patients who have repeated relapses after discontinuing anti-thyroid drugs but who continue to decline surgery or radioactive iodine. For many of these patients, therapy with small doses of methimazole (e.g., 5 mg daily) provides long-term stability. Although patients are often told that antithyroid drugs can be used for only a limited period, that notion is incorrect, provided that the patient has no long-term toxicity. Radioactive iodine therapy for a so-called hot nodule (an autonomously producing adenoma) approaches ideal therapy. A single dose of sodium iodide I 131 usually cures the hyperthyroidism. Most patients become euthyroid, and some become hypothyroid. The nodule may persist. It should be noted that some questions have been raised about the potential long-term effects of radiation from radioactive iodine on the remaining normal thyroid gland. Dopamine agonists are now the therapy of choice for most prolactinomas. In a minority of patients, the tumor disappears and does not recur after the drugs have been withdrawn. The long-acting dopamine agonists (e.g., ca­ bergoline) are easier to administer and have fewer acute side effects than their predecessors. However, some concerns have arisen about cardiac valvular problems with these newer medications. Primary medical therapy for somatotroph adenomas has become a real possibility with the advent of somatostatin agonists and GH receptor antagonists. The efficacy of these therapies can be monitored by measuring IGF1. Many TSHsecreting pituitary adenomas are also treated successfully with somatostatin analogues. However, GH- and TSHsecreting tumors do not permanently disappear. Surgery remains the therapy of choice when treatment is needed for primary hyperparathyroidism. Cinacalcet, an activator of the calcium-sensing receptor, is effective in lowering PTH levels in the secondary hyperparathyroidism of renal failure; it also lowers the calcium-phosphate product and is used as part of dialysis treatment. However, although this agent also lowers serum calcium and PTH in primary hyperparathyroidism, it does not improve bone density. The medical therapy for Cushing’s disease is still primitive. Although recurrent Cushing’s is often treated with drugs such a ketoconazole, normalization of cortisol is

rarely achieved. Treatment of Cushing’s syndrome with a glucocorticoid antagonist such as RU-486 (mifepristone) poses a challenge for endocrinologists. The therapeutic effectiveness of this drug must be monitored clinically because it does not lower serum cortisol concentrations.

Surgery, Invasive Radiologic Therapy, and the Endocrinologist One of the most important tasks for a clinical endocrinologist is to identify and make use of excellent endocrine surgeons. The basic rule to remember about surgeons is that experience is critical. Experienced pituitary surgeons achieve better results with less morbidity than less experienced pituitary surgeons do. Even though approximately half of the thyroid operations in the United States are performed by surgeons who do fewer than 10 thyroid operations per year, experienced thyroid surgeons typically perform more complete surgeries (particularly for cancer) and with lower incidences of hypoparathyroidism and injury to the recurrent laryngeal nerves. Indeed, first-rate endocrine surgeons can remove the thyroid gland safely in Graves’ disease, remove the thyroid and perform a central and lateral neck dissection in thyroid cancer, find and remove abnormal parathyroid glands in hyperparathyroidism, safely remove a pheochromocytoma or functioning adrenal adenoma endoscopically, or locate and remove a small corticotroph adenoma. The following are several do’s and don’ts with respect to endocrine surgery: • Do discuss the various surgical options with your patients, and, when indicated, do help in the management of these cases both preoperatively and postoperatively. Don’t be reluctant to let the surgeon know which operation you prefer. Do help the patient and the surgeon decide between a hemithyroidectomy and a total thyroidectomy for a patient with a thyroid nodule. • Do pre-treat your hyperthyroid patients with antithyroid drugs before thyroidectomy. Do prescribe preoperative iodine drops (to decrease thyroid vascularity) before surgery for Graves’ disease, even if the surgeon does not. Do prescribe and adjust the preoperative β-blocker when a hyperthyroid patient who is allergic to anti-thyroid drugs requires a thyroidectomy. Do take responsibility for prescribing and titrating α-blockade (and sometimes β-blockade) before surgery for pheochromocytoma, if indicated. • Do discuss the role of adrenal-sparing endoscopic surgery with the surgeon in cases of MEN2 patients with pheochromocytomas. Do diagnose and manage secondary adrenal insufficiency after successful surgery for corticotroph adenomas or cortisol-secreting adrenal adenomas, and do monitor the hypothalamicpituitary-adrenal axis for recovery. • Do take responsibility for managing postoperative hypoparathyroidism, because endocrinologists usu­ ally have the most experience dealing with this condition. The following are several situations in which I believe surgery is underutilized: • When amiodarone causes protracted hyperthyroidism or the hyperthyroidism causes major cardiac problems, an expeditious thyroidectomy can be life-saving. We are still uncertain about whether prolonged prednisone therapy for amiodarone-induced destructive thyroiditis is safer than an expeditious operation, but surgery should always be considered as an option.

26   Clinical Endocrinology: A Personal View • In too many cases, unsuccessful surgery for pituitary Cushing’s disease is followed by prolonged unsuccessful medical therapy (usually with ketoconazole). The unfortunate result is that patients are exposed to the deleterious effects of glucocorticoid excess for a prolonged period. Bilateral endoscopic adrenalectomy is almost always definitive therapy for hypercortisolemia and results in surgically induced Addison’s disease. When hypercortisolemia from ectopic Cushing’s syndrome is life-threatening and the primary tumor cannot be excised, a bilateral endoscopic adrenalectomy can dramatically improve the patient’s quality of life. On the other hand, if the prognosis from the primary tumor is poor, this is not a reasonable approach. • Many endocrinologists either do not understand or do not believe that an “indeterminate” thyroid follicular neoplasm (FNA) is actually a “suspicious” FNA with a malignancy risk of 10% to 40% (usually about 20%). If a hot nodule has been excluded by serum TSH measurement or radioiodine scanning, surgery to remove the nodule is the proper treatment. Some tumors can be treated with interventional radiologic procedures rather than invasive surgery. These procedures include alcohol ablation; radiofrequency ablation (RFA) or laser ablation of hot thyroid nodules; alcohol or laser ablation of parathyroid adenomas; alcohol or RFA ablation of recurrent nodal disease from papillary thyroid carcinoma; arterial ablation of mediastinal parathyroid adenomas; RFA of metastases to the adrenals and, rarely, for functioning adrenal tumors; and alcohol ablation, thermoablation, RFA, or chemoembolization of hepatic metastases from endocrine tumors. Embolization of bony metastases from thyroid cancer, and of spinal metastases in particular, is often performed before surgical resection. In all situations, the potential morbidity of these procedures and the local expertise must be weighed against the morbidity of surgery. For example, in some patients alcohol ablation of parathyroid tumors causes severe pain and scarring, but this may depend on the expertise and experience of the physician. Laser ablation is not currently available in the United States, and it is still not clear when these nonsurgical procedures should be used.

Turning Negatives into Positives Observant endocrinologists have noted the negative side effects of some drugs and then taken advantage of them for therapy: • Lithium inhibits the release of thyroid hormone and can cause hypothyroidism. It has occasionally been used to treat Graves’ hyperthyroidism. • Thiazides impair calcium excretion and can elevate the serum calcium level. They have been used to treat the hypercalciuria of primary hyperparathyroidism. The minimal rise in serum calcium may cause a possibly beneficial fall in PTH concentration. It is also of interest that thiazide diuretics are associated with increased bone density in the general population. • Demeclocycline and lithium can cause mild diabetes insipidus. These drugs have a beneficial effect on chronic hyponatremia resulting from the syndrome of inappropriate secretion of antidiuretic hormone (SIADH). • Cholestyramine inhibits the absorption of levothyroxine and can worsen hypothyroidism if administered conjointly with levothyroxine. However, hyperthyroidism improves more quickly when

cholestyramine is added to the anti-thyroid drug regimen, because it inhibits the enterohepatic circulation of thyroid hormone. • Amiodarone is the most important cause of druginduced hyperthyroidism. At the same time, amiodarone is also the most potent currently available inhibitor of T4-to-T3 conversion. Because of this property, amiodarone may be added to antithyroid drugs to treat severe hyperthyroidism more quickly. • The abortifacient RU-486 blocks both the progesterone and the cortisol receptor. It may be useful in the emergency treatment of endogenous Cushing’s syndrome or severe glucocorticoid excess. • Recognition of the downregulation of luteinizing hormone and FSH that occurs after administration of exogenous gonadotropin-releasing hormone (GnRH) has led to the development of GnRH analogues that inhibit gonadotropin release.

THE ENDOCRINOLOGIST AS ONCOLOGIST Although it is easy to frighten patients, it takes time, patience, and sensitivity to reassure them. For example, the clinician could say to a patient, “Your thyroid (or adrenal) nodule could be a cancer.” But for nodules that are obviously not serious malignancies, it would be far better and equally honest to tell the patient, “You have a thyroid nodule. Ninety percent of thyroid nodules are benign, and most thyroid cancers are not life-threatening. We will do a biopsy to help us determine whether surgery is necessary.” Although this approach requires a bit of modification for adrenal nodules, it would still be reasonable to say, “The vast majority of adrenal nodules are benign, and adrenal cancers are rare.” With most innocent or nonaggressive thyroid cancers, the rule is to think and act like an endocrinologist. With more aggressive thyroid and other endocrine cancers, a good rule is to think like an oncologist and to refer the patient to one of them if necessary. In most cases, sensitive tumor markers measured in a simple blood test (thyroglobulin) can tell us whether patients with well-differentiated thyroid cancer are cured. For example, it is relatively common for patients who have undergone total thyroidectomy, with or without nodal dissection for intrathyroidal papillary thyroid carcinoma with nodal metastatic disease, to have persistent, mildly elevated serum thyroglobulin at baseline or after TSH stimulation. But even if these patients are not cured, they should be told that the prognosis is usually excellent. It is important to emphasize to them that an abnormal thyroglobulin concentration is not lethal. Although thyroglobulin persistence is indicative of residual tumor, the remaining tumor is rarely lethal and in many patients never becomes manifest. You might say something like, “We will look for residual disease, and we will remove as much as we can. But even if we do not find anything, you will be fine.” Few things cost as little but do as much as a strong dose of reassurance. Abnormal concentrations of calcitonin and carcinoembryonic antigen after surgery for medullary thyroid carcinoma are of greater concern, because this tumor can have a worse prognosis. Many patients with advanced medullary thyroid carcinoma die of their disease. On the other hand, many patients with persistent modest calcitonin elevations live symptom free for decades despite persistent disease. Although the serum calcitonin concentration roughly

Clinical Endocrinology: A Personal View    27

correlates with tumor volume, it is not a good indicator of a patient’s prognosis. The doubling time of serum calcitonin is a better prognostic indicator: the shorter the doubling time, the worse the prognosis.

Know Your Cancer It is very important to understand the course, natural history, and manifestations of endocrine malignancies: • Younger patients with radioactive iodine–refractory papillary or follicular thyroid cancer metastatic to the lungs may have indolent disease that remains unchanged over decades. If these lesions are negative on fluorodeoxyglucose positron emission tomography (FDG-PET) and the patient is asymptomatic, careful observation is often the most appropriate course of action. Surgery cannot cure diffuse pulmonary disease, and current chemotherapy regimens are not indicated for stable or slow-growing disease. Careful observation may also be the best approach in older, asymptomatic patients with slow-growing PETpositive or -negative pulmonary metastases. • Papillary and follicular thyroid cancers rarely cause pleural effusions even when they are metastatic to the lung. In a patient with well-differentiated thyroid cancer, extensive pleural effusions in the absence of sizable pulmonary metastases suggest the presence of another malignancy or diagnosis. • In general, most thyroid cancers (with the important exception of anaplastic thyroid carcinoma) do not kill in a systemic way. Even in advanced thyroid cancer, cachexia is rare until the patient is near death. We therefore recommend aggressive local surgery for important lesions, even when removal is not curative. Such resections include both tumors that invade the trachea or esophagus and a limited number of growing pulmonary lesions that threaten the major bronchi. External irradiation may be appropriate for some of these lesions. Growing and symptomatic hepatic metastases may be treated with chemoembolization, alcohol ablation, or RFA even if these procedures are not curative. When treating well-differentiated thyroid cancer, I recommend avoiding whole-brain irradiation for brain metastases whenever possible. Whole-brain irradiation often causes major brain dysfunction within 1 or 2 years. If the patient is expected to live longer than 2 years, whole-brain irradiation seems inappropriate to me. I recommend surgical resection, if possible, and focused irradiation (including proton irradiation) either in addition or as an alternative. • In some cancer patients, the major cause of morbidity is hormone excess rather than the cancer itself. Cushing’s syndrome due to ACTH production by medullary thyroid carcinoma is often not diagnosed even though it is amenable to treatment. Muscle weakness and hypokalemia are important clues to the diagnosis. Hypercalcemia from metastatic parathyroid carcinoma is often refractory to therapy. The best therapy is to try to resect as much tumor as possible. If surgery is not an option or is unsuccessful, cinacalcet in high doses may provide some benefit. • Anaplastic thyroid carcinoma is almost invariably fatal. Patients must be informed of the prognosis. Although several controlled trials are under way, successful therapy is probably years away. • Bony metastases from well-differentiated thyroid cancer are invariably lytic before therapy and may be

extensive with advanced disease. However, hypercalcemia is rare unless the tumors have shown squamous de-differentiation. Conventional bone scans may be falsely negative in patients with lytic metastases, because these scans measure deposition of isotopetagged bisphosphonate in bone. • Hypercalcemia in a patient with medullary thyroid carcinoma is more likely to be related to concomi­ tant hyperparathyroidism (MEN2) than to bony metastases.

Endocrinology-Oncology Collaboration Radioactive iodine is an effective targeted therapy for welldifferentiated thyroid cancer with radioiodine-avid metastatic disease. However, the use of radioiodine as a diagnostic or therapeutic tool is often continued long after it has ceased to be effective. If the serum thyroglobulin concentration is elevated and a high-dose radioactive iodine scan is not informative, then repeated high-dose scans or low-dose scans will also be futile. Patients are frequently referred to me with metastatic thyroid cancer and elevated serum thyroglobulin levels in whom the only localizing tests have been multiple negative 131I scans. Oncologists know, and endocrinologists need to realize, that conventional imaging is necessary to identify structural disease in this situation. An ultrasound study in skilled hands identifies most neck metastases, and a CT or MRI of the neck identifies almost all of the others. A high-resolution CT of the chest identifies almost all pulmonary metastases and significant intrathoracic nodal disease. If inoperable progressive disease is present, the presence and intensity of FDG uptake may help predict both the lack of 131I efficacy and a worse prognosis. In the case of rapidly progressive or symptomatic radioactive iodine–refractory disease, therapeutic trials or available targeted chemotherapy (currently tyrosine kinase inhibitors) should be considered. Targeted chemotherapy for advanced thyroid cancer using tyrosine kinase inhibitors has had notable but temporary success during the past 5 years.15 Patients with pulmonary or other metastases that are refractory to radioactive iodine may or may not need additional therapy. For FDG-PET–negative lesions that are asymptomatic and stable or growing slowly, observation may be best. If the lesion is growing more quickly or is FDG-PET positive, these difficult questions often require the input of an oncologist: What is the likely life expectancy? Is simple observation the appropriate recommendation? Should targeted chemotherapy be considered? If so, when should it start? There is a growing need for endocrine oncologists or oncologists with expertise in treating endocrine malignancies. Oncologic expertise is essential in the treatment of thyroid lymphomas, anaplastic thyroid carcinoma, advanced well-differentiated and medullary thyroid carcinoma requiring therapy, metastatic ACC, metastatic pheochromocytoma, islet cell malignancies, and parathyroid carcinoma. It is important for endocrinologists to collaborate with oncologists in treating these endocrine malignancies, because endocrinologists are more familiar with certain endocrine issues that arise during the course of treatment: • Endocrinologists know that tyrosine kinase inhibitors increase the requirement for thyroid hormone, thus raising the serum TSH (and potentially causing tumor growth) in patients with well-differentiated thyroid cancer.

28   Clinical Endocrinology: A Personal View • Endocrinologists know that mitotane is a potentially important adjunctive therapy for ACC but that it may temporarily or permanently damage the remaining adrenal gland after unilateral adrenalectomy for ACC, and it may accelerate the metabolism of cortisol, thereby causing an increased glucocorticoid requirement and possibly acute adrenal insufficiency. • Endocrinologists know that the catecholamine excess of metastatic pheochromocytoma often must be treated with α-blockers or inhibitors of catecholamine biosynthesis, either chronically, before surgery, or before therapies associated with release of catecholamines. The metastases themselves require local therapy if they are symptomatic, and they require systemic targeted therapy when resection, radiation therapy, or ablative therapy is not possible or not successful. • Endocrinologists are skilled at managing the refractory hypoglycemia of insulinomas that are metastatic and unresectable. • Endocrinologists are most knowledgeable about treating the hypercalcemia of metastatic parathyroid carcinoma with cinacalcet and other agents. There are many unresolved therapeutic dilemmas concerning endocrine malignancies, particularly when disease is advanced. However, even initial therapeutic decisions can be problematic and might benefit from collaborative efforts with oncologists. • Pancreatic neuroendocrine tumors are the leading cause of death in MEN1, yet we lack an effective diagnostic and therapeutic approach to these tumors. • Patients with parathyroid carcinoma require a wide surgical excision and often have a guarded prognosis. However, there is no consensus as to whether pro­ phylactic postoperative external beam irradiation improves the prognosis. • It is uncertain whether aggressive therapy (local irradiation and chemotherapy) improves the outcome for patients who have ACC without metastatic disease at presentation. Advances in molecular biology are likely to have an important impact on the evaluation and treatment of endocrine as well as nonendocrine neoplasms. Current evidence suggests that the presence of mutated oncogenes in “follicular neoplasms” may help direct the surgeon toward a bilateral rather than a unilateral thyroidectomy. The presence of BRAF proto-oncogene mutations in papillary thyroid carcinoma may predict a worse prognosis and a lack of response to radioactive iodine. Improved ability to predict tumor behavior and to target therapy based on mutated oncogenes is among the goals for the future.

THE ENDOCRINOLOGIST AS EDUCATOR AND STUDENT Clinical endocrinology requires lifelong learning. The clinician must keep up-to-date with current research, maintain a continuous dialogue with colleagues, know the literature, and take time to think about complicated cases. Only clinicians who understand the subject can teach it clearly and efficiently to students, fellows, colleagues, and patients. If a description or explanation seems fuzzy, it is probably either incomplete or incorrect. Information contained in textbook chapters or on-line textbooks can be helpful, but secondary sources should always be used in conjunction with original data and original publications.

The Internet has both facilitated and complicated patient education. Some Internet information is excellent, some misleading, some sensationalized, and some completely wrong. Clinical endocrinologists must acquire familiarity with the relevant web sites so that they can advise their patients about which ones are most reliable. All clinical endocrinologists should provide handouts for their patients containing important information about their disease and its treatment. There are some excellent handouts published by professional endocrine societies, but it is often best to explain the material to your patients in your own words.

FUTURE DIRECTIONS AND CONSIDERATIONS We have come to realize that the body does not act as a single, homogeneous unit with respect to hormone effects, and it may be possible to target additional hormone receptors and pathways for therapeutic purposes. • When the peripheral conversion of T4 to T3 is inhibited, the pituitary remains euthyroid with unchanged TSH production, but the periphery may become hypothyroid (exposed to a low T3 concentration). • SERMs permit differential estrogen effects in certain tissues, thereby leading to important therapies for prevention of osteoporosis and breast cancer while avoiding potential negative estrogen effects. Similarly, thyroid hormone analogues have been developed that have very favorable effects on lipid profiles without causing tachycardia. Thyroid analogues capable of TSH suppression without other thyroid hormone actions may be feasible. • Appropriate targeting of glucocorticoid nuclear activating pathways may eventually be the basis for a therapy that has anti-inflammatory properties without causing immunosuppression. • If regional Cushing’s syndrome resulting from activation of cortisone to cortisol in the liver by 11β-hydroxysteroid dehydrogenase 1 is found to play a role in central obesity and the metabolic syndrome, it is possible that appropriate therapies might follow. • In Graves’ disease, the thyroid gland is an innocent bystander being stimulated by immunoglobulins (TRAb). A magic bullet for Graves’ disease would be a therapy that turns off these specific immunoglobulins selectively and safely. • Therapy for severe Graves’ ophthalmopathy (glucocorticoids, orbital radiation, and/or orbital decompression) has been essentially static for decades. New insights and innovations are sorely needed. • Genome-wide association studies may yet uncover new, unimagined pathways with important diagnostic, preventive, and therapeutic consequences for clinical endocrinology. Consider for a moment how far we have come in the practice of endocrinology. Forty years ago, we were still debating whether an elevated blood glucose concentration contributes to diabetic complications. Diabetes was monitored by spot urine glucose determinations. It was almost impossible to diagnose subtle endocrine dysfunction. It was thought that ACTH-dependent Cushing’s disease was primarily a hypothalamic disorder. The only way to image the pituitary gland itself was to subject the patient to an extremely painful pneumoencephalogram. Immunoassays were still in their infancy: TSH

Clinical Endocrinology: A Personal View    29

measurements were not yet part of the evaluation for hyperthyroidism, and we could not yet detect serum calcitonin when the concentration was less than 1000  pg/ mL. The only treatment for osteoporosis was estrogen or, occasionally, androgens. Thyroid nodules were discovered only by palpation. Only a few major centers offered thyroid biopsies, and these were performed by surgeons with large cutting needles. The promise of molecular biology was precisely that, a promise only. Pituitary hormones were extracted from pituitary glands rather than produced as recombinant human hormones. The concept of oncogenes and tumor suppressor genes was unknown. There was no Internet, and there were no guidelines. Much of the excitement and satisfaction I have experienced in being a clinical endocrinologist during the past 40 years has come from seeing all these changes unfold, integrating them into my own practice, learning this new science and technology, thinking about solutions to complicated clinical problems, and finding new clinical conditions. I can’t wait to see what the future holds.

REFERENCES 1. Tomlinson JW, Walker EA, Bujalska IJ, et al. 11Beta-hydroxysteroid dehydrogenase type 1: a tissue-specific regulator of glucocorticoid response. Endocr Rev. 2004;25:831-866. 2. Pallais JC, Kifor O, Chen YB, et al. Acquired hypocalciuric hypercalcemia due to autoantibodies against the calcium-sensing receptor. N Engl J Med. 2004;351:362-369.

3. Service GJ, Thompson GB, Service FJ, et al. Hyperinsulinemic hypoglycemia with nesidioblastosis after gastric-bypass surgery. N Engl J Med. 2005;353:249-254. 4. Christ E, Wild D, Forrer F, et al. Glucagon-like peptide-1 receptor imaging for localization of insulinomas. J Clin Endocrinol Metab. 2009;94:4398-4405. 5. Funder JW. The role of aldosterone and mineralocorticoid receptors in cardiovascular disease Am J Cardiovasc Drugs. 2007;7:151-157. 6. Pope JE. Hypertension, NSAID, and lessons learned. J Rheumatol. 2004;31:1035-1037. 7. Weitzman SA, Stossel TP, Harmon DC, et al. Antineutrophil autoantibodies in Graves’ disease: implications of thyrotropin binding to neutrophils. J Clin Invest. 1985;75:119-123. 8. Jacobus CH, Holick MF, Shao Q, et al. Hypervitaminosis D associated with drinking milk. N Engl J Med. 1992;326:1173-1177. 9. Neumann HP, Vortmeyer A, Schmidt D, et al., Evidence of MEN-2 in the original description of classic pheochromocytoma. N Engl J Med. 2007;357:1311-1315. 10. Osler W. Recent advances in medicine. Science. 1891;17:170-171. 11. Gussekloo J, van Exel E, de Craen AJ, et al. Thyroid status, disability and cognitive function, and survival in old age. JAMA. 2004;292:25912599. 12. Park L, Wexler D. Update in diabetes and cardiovascular diseases: synthesizing the evidence from recent trials of glycemic control to prevent cardiovascular disease. Curr Opin Lipidol. 2010;21:8-14. Epub 2009 Oct 13. 13. Panicker V, Saravanan P, Vaidya B, et al. Common variation in the DIO2 gene predicts baseline psychological well-being and response to combination thyroxine plus triiodothyronine therapy in hypothyroid patients. J Clin Endocrinol Metab. 2009;94:1623-1629. 14. Hamrahian AH, Oseni TS, Arafah BM. Measurements of serum free cortisol in critically ill patients. N Engl J Med. 2004;350:16291638. 15. Schlumberger M, Sherman SI. Clinical trials for progressive differentiated thyroid cancer: patient selection, study design, and recent advances. Thyroid. 2009;19:1393-1400.

Evolution of Peptide Hormones and Their Functions,  31 Steps in Expression of a Protein-Encoding Gene,  31 Subcellular Structure of Cells That Secrete Protein Hormones,  32 Intracellular Segregation and Transport of Polypeptide Hormones,  33 Processes of Hormone Secretion,  36 Structure of a Gene Encoding a Polypeptide Hormone,  37 Regulation of Gene Expression,  38 Biologic Diversification,  43

CHAPTER CHAPTER 3  Genetic Control of Peptide Hormone Formation JOEL F. HABENER

Advances in the fields of molecular and cellular biology have provided insights into the mechanistic workings of cells. Recombinant deoxyribonucleic acid (DNA) technology and sequencing (decoding) of the genomes of human, mouse, and several other species make it possible to analyze the precise structure and function of DNA. Discovery of the unique biochemical and structural properties of DNA provided the conceptual framework with which to begin a systematic investigation of the origins, development, and organization of life forms.1 Sequencing of the human and mouse genomes was accomplished in 2003 and 2004, and annotation of the encoded information is nearing completion. A complete blueprint of the structure and organization of all expressed genes has illuminated the basis of genetically determined diseases. Within the next decade, genotyping of individuals shortly after birth likely will be possible and will provide information about the relative risks of developing these diseases. Therapeutic approaches for the correction of genetic defects by techniques of gene replacement are likely to become a reality. The polypeptide hormones constitute a critically important and diverse set of regulatory molecules encoded by 30

the genome; their functions are to convey specific information among cells and organs. This type of molecular communication arose early in the development of life and evolved into a complex system for the control of growth, development, and reproduction and maintenance of metabolic homeostasis. These hormones, including the many chemokines and cytokines primarily involved in regulation of the immune system, consist of more than 400 small proteins, ranging from as few as 3 amino acids (thyrotropin-releasing hormone [TRH]) to 192 amino acids (growth hormone [GH]). These polypeptides function as hormones, whose actions on distant organs are mediated by way of their transport through the bloodstream, and as local cell-to-cell communicators (Fig. 3-1). The latter function of the polypeptide hormones is exemplified by their elaboration and secretion within neurons of the central, autonomic, and peripheral nervous systems, where they act as neurotransmitters, and in leukocytes, where they modulate immune responses. The multiple modes of expression of the polypeptide hormone genes have aroused great interest in the specific functions of these peptides and the mechanisms of their synthesis and release.

Genetic Control of Peptide Hormone Formation   31 Endocrine

Paracrine Brain Pancreatic islets

Pituitary gland

δ Cell

α Cell

Hormones Bloodstream

The genes encoding polypeptide hormones, particularly those for regulatory peptides, evolved early in the development of life and initially fulfilled the function of cell-to-cell communication to cope with problems concerning nourishment, growth, development, and reproduction. As specialized organs connected by a circulatory system developed during evolution, similar or identical gene products became hormones for purposes of organ-to-organ communication.

Hormones

STEPS IN EXPRESSION OF A PROTEIN-ENCODING GENE

Thyroid gland Target organ Neuroendocrine

Neurotransmitter

Neurohypophyseal portal system Hormones

Presynaptic Hormones

Postsynaptic Neurons

Figure 3-1 Modes of polypeptide hormone expression. The peptide hormones are expressed in at least four ways in fulfilling their functions as cellular messenger molecules: (1) endocrine mode, for purposes of communication among organs (e.g., pituitary-thyroid axis); (2) paracrine mode, for communication among adjacent cells, often located within endocrine organs; (3) neuroendocrine mode, for synthesis and release of peptides from specialized peptidergic neurons for action on distant organs through the bloodstream (e.g., neuroendocrine peptides of hypothalamus); and (4) neurotransmitter mode, for action of peptides in concert with classic amino acid–derived aminergic transmitters in the neuronal communication network. Identical polypeptides are often used in the nervous system as both neuroendocrine hormones and neurotransmitters. In some instances, the same gene product is used in all four modes of expression.

This chapter reviews the diverse structures of genes encoding peptide hormones and the mechanisms that govern their expression. The synthesis of nonpeptide hormones (e.g., catecholamines, thyroid hormones, steroid hormones) involves the action of many enzymes and expression of multiple genes, which are discussed in other chapters of this text.

EVOLUTION OF PEPTIDE HORMONES AND THEIR FUNCTIONS Peptide hormones arose early in the evolution of life. Polypeptides that are structurally similar to mammalian peptides are present in lower vertebrates, insects, yeasts, and bacteria.2 An example of the early evolution of regulatory peptides is the α-factor (i.e., mating pheromone) of yeast, which is similar in structure to gonadotropinreleasing hormone (GnRH).3 The oldest member of the cholecystokinin-gastrin family of peptides appeared at least 500 million years ago in the protochordate Ciona intestinalis (sea squirt).4

The steps involved in transfer of information encoded in the polynucleotide language of DNA to the poly-aminoacid language of biologically active proteins involve gene transcription, post-transcriptional processing of ribonucleic acids (RNAs), translation, and post-translational processing of the proteins. The expression of genes and protein synthesis can be considered in terms of several major processes, any one or more of which may serve as specific control points in the regulation of gene expression (Fig. 3-2). • Rearrangements and transpositions of DNA segments. These processes occur over eons in evolution, with the exception of uncommon mechanisms of somatic gene rearrangements such as rearrangements in the immunoglobulin genes occurring during the lifetime of an individual. • Transcription. Synthesis of RNA results in the formation of RNA copies of the two gene alleles and is catalyzed by the basal RNA polymerase II–associated transcription factors. • Post-transcriptional processing. Specific modifications of RNA include the formation of messenger RNA (mRNA) from the precursor RNA by way of excision and rejoining of RNA segments (introns and exons) and the modification of the 3′ end of the RNA by polyadenylation and of the 5′ end by addition of 7-methylguanine caps. • Translation. Amino acids are assembled by base pairing of the nucleotide triplets (anticodons) of the specific “carrier” amino-acylated transfer RNAs to the corresponding codons of the mRNA bound to polyribosomes and are polymerized into polypeptide chains. • Post-translational processing and modification. Final steps in protein synthesis may involve one or more cleavages of peptide bonds, which result in the conversion of biosynthetic precursors (prohormones) to intermediate or final forms of the protein; derivatization of amino acids (e.g., glycosylation, phosphorylation, acetylation, myristoylation); and folding of the processed polypeptide chain into its native conformation. Each of the specific steps of gene expression requires the integration of precise enzymatic and other biochemical reactions. These processes have developed to provide high fidelity in reproduction and transmission of the encoded information and to supply control points for genes that direct the expression of the specific phenotype of cells. Post-translational processing of proteins creates diversity in gene expression through modifications of the protein. Although the functional information contained in a protein is ultimately encoded in the primary amino acid sequence, the specific biologic activities are a consequence of the secondary, tertiary, and quaternary structures of the polypeptide. Given the wide range of possible specific modifications of the amino acids (e.g., glycosylation,

32   Genetic Control of Peptide Hormone Formation Nucleus Chromatin (DNA + protein)

Nucleosome

DNA

RNA polymerase mRNA precursor

Transcription

Excision of introns

Pre-mRNA

Rejoining Post-transcriptional processing mRNA “Cap”

AAAAA3´

Me7 GPPP

mRNA

Cytoplasm

Translation

tRNA, amino acids mRNA Ribosomes Golgi region Cisterna of endoplasmic reticulum Pre-prohormone Cotranslational

Secretion

CHO Prohormone

Preprotein

Mature hormone

Post-translational

Modification

Protein Transport

Posttranscriptional processing —Presecretory —Postsecretory

Biologic action Figure 3-2  Cellular synthesis of polypeptide hormones. Steps that take place within the nucleus include transcription of genetic information into a messenger ribonucleic acid precursor (pre-mRNA) followed by post-transcriptional processing, which includes RNA cleavage, excision of introns, and rejoining of exons, resulting in formation of mRNA. Ends of mRNA are modified by addition of methylguanosine caps at the 5′ end and poly(A) tracts at the 3′ ends. The cytoplasmic mRNA is assembled with ribosomes. Amino acids, carried by amino-acylated transfer RNAs (tRNAs), are then polymerized into a polypeptide chain. The final procesess in protein synthesis take place during growth of the nascent polypeptide chain (cotranslational) and after release of the completed chain (post-translational). They include proteolytic cleavages of the polypeptide chain (conversion of pre-prohormones or prohormones to hormones), derivatizations of amino acids (e.g., glycosylation, phosphorylation), and cross-linking and assembly of the polypeptide chain into its conformed structure. Posttranslational synthesis and processing of a typical secreted polypeptide require vectorial or unidirectional transport of the polypeptide chain across the membrane bilayer of the endoplasmic reticulum, resulting in sequestration of the polypeptide in the cisterna of the endoplasmic reticulum, a first step in the export of proteins destined for secretion from the cell (see Fig. 3-6). Most translational processing occurs within the cell (presecretory); in some instances, it occurs outside the cell, when further proteolytic cleavages or modifications of the protein take place (postsecretory). CHO, carbohydrate.

phosphorylation, acetylation, amidation, lipidation, sulfation),5 any one of which may affect the conformation or function of the protein, a single gene may ultimately encode a wide variety of specific proteins as a result of post-translational processes. Polypeptide hormones are synthesized in the form of larger precursors that appear to fulfill several functions in biologic systems (Fig. 3-3), including intracellular trafficking, by which the cell distinguishes among specific classes of proteins and directs them to their sites of action, and the generation of multiple biologic activities from a common genetically encoded protein by regulated or cellspecific variations in the post-translational modifications (Fig. 3-4). All peptide hormones and regulatory peptides studied contain signal or leader sequences at the amino-terminus. These hydrophobic, helical sequences recognize specific sites on the membranes of the rough endoplasmic reticulum (ER), which results in the transport of nascent polypeptides into the secretory pathway of the cell (see Figs. 3-2 and 3-3).6 The consequence of the specialized signal sequences of the precursor proteins is that proteins destined for secretion are selected from a great many other cellular proteins for sequestration and subsequent

packaging into secretory granules and export from the cell. Most smaller hormones and regulatory peptides are produced as a consequence of post-translational cleavages of the precursors within the Golgi complex of secretory cells.

SUBCELLULAR STRUCTURE OF CELLS THAT SECRETE PROTEIN HORMONES Cells whose principal functions are the synthesis and export of proteins contain highly developed, specialized subcellular organelles for translocating secreted proteins and packaging them into secretory granules. The subcellular pathways used in protein secretion were elucidated largely through the early efforts of Palade (reviewed by Jamieson).7,8 Secretory cells contain an abundance of ER, Golgi apparatus, and secretory granules (Fig. 3-5). The proteins that are to be secreted from the cells are transferred during their synthesis into these subcellular organelles, which transport the proteins to the plasma membrane. Protein secretion begins with translation of the mRNA encoding the precursor of the protein on the rough ER, which consists of polyribosomes attached to elaborate

Genetic Control of Peptide Hormone Formation   33 Prehormone Signal

INTRACELLULAR SEGREGATION AND TRANSPORT OF POLYPEPTIDE HORMONES

Apoprotein (Bioactive)

Pre-prohormone (Polyprotein) Signal

Cryptic

Bioactive

Spacer

Bioactive

Figure 3-3 Two configurations of precursors of polypeptide hormones. Diagrams represent the polypeptide backbones of protein sequences encoded in messenger RNA (mRNA). One form of precursor consists of the amino-terminal signal, or presequence, followed by the apoprotein portion of the polypeptide that needs no further proteolytic processing for activity. A second form of precursor is a pre-prohormone that consists of the N-terminal signal sequence followed by a polyprotein, or prohormone, sequence made up of two or more peptide domains linked together that are subsequently liberated by cleavages during post-translational processing of the prohormone. The reason for synthesis of polypeptide hormones in the form of precursors is only partly understood. The N-terminal signal sequences function in the early stages of transport of polypeptide into the secretory pathway. Prohormones, or polyproteins, often provide a source of multiple bioactive peptides (see Fig. 3-4). However, many prohormones contain peptide sequences that are removed by cleavage and have no known biologic activity (cryptic peptides). Other peptides may serve as spacer sequences between two bioactive peptides (e.g., the C peptide of proinsulin). When a bioactive peptide is located at the carboxyl-terminus of the prohormone, the N-terminal prohormone sequence may simply facilitate the cotranslational translocation of polypeptide in the endoplasmic reticulum (see Fig. 3-6).

Specific amino acid sequences encoded in the proteins serve as directional signals for the sorting of proteins within subcellular organelles.6,9,10 A typical eukaryotic cell synthesizes an estimated 5000 different proteins during its life span. Although these proteins are synthesized by a common pool of polyribosomes, each protein is directed to a specific location within the cell, where its biologic function is expressed. For example, specific groups of proteins are transported into mitochondria, into membranes, into the nucleus, or into other subcellular organelles, where they serve as regulatory proteins, enzymes, or structural proteins. A subset of proteins including immunoglobulins, serum albumin, blood coagulation factors, and protein and ProInsulin B

C

A

ProSomatostatin

SS-28 SS-14

ProGlucagon

IP-I IP-II Glucagon GLP-I GLP-II

Pro-Opiomelanocortin γ-MSH

membranous saccules that contain cavities (cisternae). The newly synthesized, nascent proteins are discharged into the cisternae by transport across the lipid bilayer of the membrane. Within the cisternae of the ER, proteins are carried to the Golgi complex by mechanisms that are incompletely understood. The proteins gain access to the Golgi complex by direct transfer from the cisternae, which are in continuity with the membranous channels of the Golgi complex, or by way of shuttling vesicles known as transition elements (see Fig. 3-5). Within the Golgi complex, the proteins are packaged into secretory vesicles or secretory granules that bud from the Golgi stacks in the form of immature granules. Immature granules undergo maturation through condensation of the proteinaceous material and application of a specific coat around the initial Golgi membrane. On receiving the appropriate extracellular stimuli (i.e., regulated pathway of secretion), the granules migrate to the cell surface and fuse to become continuous with the plasma membrane, which results in the release of proteins into the extracellular space, a process known as exocytosis. The second pathway of intracellular transport and secretion involves the transport of proteins contained within secretory vesicles and immature secretory granules (see Fig. 3-5). Although the use of this alternative vesicle-mediated transport pathway remains to be demonstrated conclusively (it is typically considered to be a constitutive, or unregulated, pathway), extracellular stimuli may modulate hormone secretion differently depending on the pathway of secretion. For example, in the parathyroid gland and in the pituitary cell line derived from corticotropic cells (AtT20), newly synthesized hormone is released more rapidly than hormone synthesized earlier. This finding suggests that the newly synthesized hormone may be transported by way of a vesicle-mediated pathway without incorporation into mature storage granules.

ProEnkephalin

M

δ-MSH CLIP

M

M

ProPressophysin ADH Neurophysin

M

γ-LPH

M

β-Endorphin

L

M

Glycopeptide

ProParathyroid hormone PTH ProGastrin

ProCalcitonin

Pro-CGRP

G-34 G-17

Calcitonin C-peptide

Calcitonin gene-related peptide

Figure 3-4  Primary structures of some prohormones. The shaded areas of prohormones denote regions of sequence that constitute known biologically active peptides after post-translational cleavage from the prohormone. Sequences indicated by hatching denote regions of precursor that alter the biologic specificity of that region of precursor. For example, the precursor contains the sequence of γ-melanocyte-stimulating hormone (γ-MSH), but when the latter is covalently attached to the CLIP peptide, it constitutes adrenocorticotropic hormone (ACTH). Somatostatin-28 (SS-28) is an amino-terminally extended form of somatostatin-14 (SS-14) that has higher potency than SS-14 on certain receptors. The neurophysin sequence linked to the carboxyl-terminus of vasopressin (ADH) functions as a carrier protein for the ADH hormone during its transport down the axon of neurons in which it is synthesized. The precursor proenkephalin represents a polyprotein that contains multiple similar peptides within the sequence of metenkephalin (M) or leu-enkephalin (L). Procalcitonin and procalcitonin gene–related product (CGRP) share identical N-terminal sequences but differ in their C-terminal regions as a result of alternative splicing during post-transcriptional processing of the RNA precursor. γ-LPH, γ-lipotropin; GLP, glucagon-like peptide; IP, intervening peptide.

34   Genetic Control of Peptide Hormone Formation

Nucleus

Capillary lumen Transition elements

Cisternae

1

2

3

Secretory vesicles

Immature secretory granules 4

Mature secretory granules Plasma membrane RER SER/Golgi Lysosome Figure 3-5  Subcellular organelles involved in transport and secretion of polypeptide hormones or other secreted proteins within a protein-secreting cell. (1) Synthesis of proteins on polyribosomes attached to rough endoplasmic reticulum (RER) and vectorial discharge of proteins through the membrane into the cisterna. (2) Formation of shuttling vesicles (transition elements) from endoplasmic reticulum followed by their transport to and incorporation by the Golgi complex. (3) Formation of secretory granules in the Golgi complex. (4) Transport of secretory granules to the plasma membrane, fusion with the plasma membrane, and exocytosis resulting in the release of granule contents into the extracellular space. Notice that secretion may occur by transport of secretory vesicles and immature granules or by transport of mature granules. Some granules are taken up and hydrolyzed by lysosomes (crinophagy). Golgi, Golgi complex; SER, smooth endoplasmic reticulum. (From Habener JF. Hormone biosynthesis and secretion. In: Felig P, Baxter JD, Broadus AE, et al, eds. Endocrinol Metab. New York, NY: McGraw-Hill, 1981:29-59.)

polypeptide hormones is specifically designed for export from the cell. This process of directional transport of proteins involves sophisticated informational signals. Because the information for these translocation processes must reside wholly or in part within the primary structure or in the conformational properties of the protein, sequential post-translational modifications may be crucial for determining the specificity of protein function. Continued investigations of protein sorting and trafficking in cells have revealed increased complexities beyond the simple paradigm illustrated in Figure 3-5.11 Sequential sorting of proteins to their final destinations, whether they are exported (secreted) from the cells or targeted to a subcellular compartment or organelle, takes place not only in the Golgi apparatus but before then in the ER, and afterward in endosomes and tubulosaccules.12 Each of the approximately 5000 proteins expressed in a given cell contains a specific targeting signal that is responsible for directing the protein to its final destination. These targeting signals consist of short stretches of amino acids in the proteins that serve as molecular “ZIP codes” to ensure their accurate delivery. Modern approaches using proteomics and bioinformatics can predict localization of proteins in cells based on the characteristics of these targeting signals.13

Signal Sequences in Peptide Prohormone Processing and Secretion The early processes of protein secretion that result in the specific transport of exported proteins into the secretory pathway have become better understood.6,10-15 Initial clues to this process came from determinations of the amino acid sequences of the proteins programmed by the cell-free translation of mRNAs encoding secreted polypeptides.16 Secreted proteins are synthesized as precursors that are

extended at their N-termini by sequences of 15 to 30 amino acids, called signal or leader sequences. Signal sequence extensions or their functional equivalents are required for targeting the ribosomal or nascent protein to specific membranes and for vectorial transport of the protein across the membrane of the ER. On emergence of the signal sequence from the large ribosomal subunit, the ribosomal complex specifically makes contact with the membrane; this results in translocation of the nascent polypeptide across the ER membrane into the cisterna as the first step in its secretory pathway. These observations initially left unanswered the question of how specific polyribosomes that translate mRNAs encoding secretory proteins recognize and attach to the ER (Fig. 3-6). Because microsomal membranes in vitro reproduce the processing activity of intact cells, it was possible to identify the macromolecules responsible for processing precursors and for translocation activities.17 The ER and the cytoplasm contain an aggregate of molecules, called a signal recognition particle complex, that consists of at least 16 proteins, including three guanosine triphosphatases to generate energy18 and a 7S RNA.6,10,19,20 This complex, or particle, binds to the polyribosomes involved in the translation of mRNAs encoding secretory polypeptides when the N-terminal signal sequence first emerges from the large subunit of the ribosome. The specific interaction of the signal recognition particle with the nascent signal sequence and the polyribosome arrests further translation of mRNA. The nascent protein remains in a state of arrested translation until it finds a high-affinity binding protein on the ER, the signal recognition particle receptor or docking protein.6 On interaction with the specific docking protein, the translational block is released, and protein synthesis resumes. The protein is then transferred across the membrane of the ER through a proteinaceous tunnel called the translocon.20

Genetic Control of Peptide Hormone Formation   35

mRNA 7S RNA Signal peptide

Translational arrest

mRNA

Signal-receptor particle (SRP) complex

Cell matrix Ribosomes

Release of translational arrest

Signal peptide

Ca2+/ Calmodulin

Signal-receptor complex CHO

Membrane

“Docking” protein

Signal peptide peptidase

Signal peptidase

Figure 3-6  Cellular events in the initial stages of synthesis of a polypeptide hormone according to the signal hypothesis. In this schema, a signal recognition particle, consisting of a complex of six proteins and an RNA (7S RNA), interacts with the amino-terminal signal peptide of the nascent polypeptide chain after approximately 70 amino acids are polymerized, arresting further growth of the polypeptide chain. The complex of the signal recognition particle and the polyribosome nascent chain remains in a state of translational arrest until it recognizes and binds to a docking protein, which is a receptor protein located on the cytoplasmic face of the endoplasmic reticular membrane. This interaction of the signal recognition particle complex with the docking protein releases the translational block, and protein synthesis resumes. The nascent polypeptide chain is discharged across the membrane bilayer into the cisterna of the endoplasmic reticulum and is released from the signal peptide by cleavage with a signal peptidase located in the cisternal face of the membrane. In this model, the signal peptide is cleaved from the polypeptide chain by signal peptidase before the chain is completed (i.e., cotranslational cleavage). The configuration of the polypeptide during transport across the membrane and the forces and mechanisms responsible for its translocation are unknown. The loop, or hairpin, configuration of the chain shown here is an arbitrary model; other models are equally possible.

At some point, near the termination of synthesis of the polypeptide chain, the N-terminal signal sequence is cleaved from the polypeptide by a specific signal peptidase located on the cisternal surface of the ER membrane. Removal of the hydrophobic signal sequence frees the protein (prohormone or hormone) so that it may assume its characteristic secondary structure during transport through the ER and the Golgi apparatus. After cleavage from the protein by a signal peptidase, the signal peptide may sometimes be further cleaved in the ER membrane to produce a biologically active peptide. For example, the signal sequence of preprolactin, comprising 30 amino acids, is cleaved by a signal peptide peptidase to yield a charged peptide of 20 amino acids that is released into the cytosol, where it binds to calmodulin and inhibits Ca2+calmodulin–dependent phosphodiesterase.21 This sequence in the directional transport of specific polypeptides ensures optimal cotranslational processing of secretory proteins, even when synthesis commences on free ribosomes. The presence of a cytoplasmic form of the signal recognition particle complex that blocks translation guarantees that the synthesis of the presecretory proteins is not completed in the cytoplasm; the efficient transfer of proteins occurs only after contact has been made with the specific receptor or docking protein on the membrane. Although the presence of signal recognition particles and docking proteins explains the specificity of the binding of ribosomes containing mRNAs encoding the secretory proteins, it does not explain the mode of translocation of the nascent polypeptide chain across the membrane bilayer.

Further dissection and analysis of the membrane have identified other macromolecules that are responsible for the transport process.6

Cellular Processing of Prohormones The signal sequences of prehormones and pre-prohormones are involved in the transport of these molecules, but the function of the intermediate hormone precursors (i.e., prohormones) is not fully understood. Conversion of prohormones to their final products begins in the Golgi apparatus. For example, the time that elapses between the synthesis of pre-proparathyroid hormone and the first appearance of parathyroid hormone correlates closely with the time required for radioautographic grains to reach the Golgi apparatus.22 Similarly, conversion of proinsulin to insulin takes place about 1 hour after the synthesis of proinsulin is complete, and processing of proinsulin to insulin and C peptide takes place during the transport within the secretory granule.23 The conversion of prohormones to hormones can also be blocked by inhibitors of cellular energy production such as antimycin A and dinitrophenol24 and by drugs that interfere with the functions of microtubules (e.g., vinblastine, colchicine).25 Translocation of the prohormone from the rough ER to the Golgi complex depends on metabolic energy and probably involves microtubules. There is no evidence that sequences specific to the prohormone contribute to or are chemically involved in transport of the newly synthesized protein from the rough ER to the Golgi apparatus, nor that they are involved in the

36   Genetic Control of Peptide Hormone Formation packaging of the hormone in vesicles or granules. Analyses of the structures of the primary products of translation of mRNAs encoding secretory proteins indicate that many of these are not synthesized in the form of prohormone intermediates (see Fig. 3-3). It remains puzzling that some secretory proteins (e.g., parathyroid hormone, insulin, serum albumin) are formed by way of intermediate precursors, whereas others (e.g., GH, prolactin, albumin) are not. Size constraints may be placed on the length of a secretory polypeptide. When the bioactivity of peptides resides at the carboxyl-termini of the precursors (e.g., somatostatin, calcitonin, gastrin), N-terminal extensions may be required to provide a sufficient “spacer” sequence to allow the signal sequence on the growing nascent polypeptide chain to emerge from the large ribosome subunit for interaction with the signal recognition particle and to provide adequate polypeptide length to span the large ribosomal subunit and the membrane of the ER during vectorial transport of the nascent polypeptide across the membrane (see Fig. 3-6). If the final hormonal product is 100 amino acids long or longer (e.g., GH, prolactin, the α- and β-subunits of the glycoprotein hormones), there may be no requirement for a prohormone intermediate. Although the exact functions of prohormones remain unknown, certain details of their cleavages have been established. Unlike the situation with prehormones, in which the amino acids at the cleavage site between the signal sequence and the remainder of the molecule (hormone or prohormone) vary from one hormone to the next, the cleavage sites of the prohormone intermediates consist of the basic amino acid lysine or arginine, or both, usually two to three in tandem. This sequence is preferentially cleaved by endopeptidases with trypsin-like activities. The family of prohormone-converting (PC) enzymes consists of at least eight specific members.26-28 The most studied of the isozymes are PC2 and PC1/3, which are responsible for the cleavages of proinsulin between the A chain/C peptide and the B chain/C peptide, respectively. A rare patient missing PC1 presented with childhood obesity, hypogonadotropic hypogonadism, and hypercortisolism and was found to have elevated proinsulin levels and presumably widespread abnormalities in neuropeptide modification.29 Targeted disruption of the PC2 gene in mice resulted in incomplete processing of proinsulin, leaving the A chain and C peptide intact.30 Proglucagon in the pancreas remained incompletely processed, indicating that PC2 is required for the formation of glucagon. As a consequence of defective PC2 activity and low levels of glucagon, the mice had severe chronic hypoglycemia. After endopeptidase cleavage, the remaining basic residues are selectively removed by exopeptidases with activity resembling that of carboxypeptidase B. When the C-terminal residue of the peptide hormone is amidated, a process that appears to enhance the stability of a peptide by conferring resistance to carboxypeptidase, specific amidation enzymes (i.e., peptide amidating monooxygenases) in the Golgi complex work in concert with the cleavage enzymes to modify the C-termini of the bioactive peptides.31,32 All proproteins and prohormones are cleaved by PC enzymatic processes within the Golgi complex of cells of diverse origins. The significance of specific cleavages of specific prohormones remains incompletely understood, as does the reason for the existence of prohormone intermediates in some but not all secretory proteins. Precursor peptides removed from the prohormones may have intrinsic biologic activities that are still unrecognized.

PROCESSES OF HORMONE SECRETION Specific extracellular stimuli control the secretion of polypeptide hormones. These stimuli consist of changes in homeostatic balance; the hormonal products released in response to the stimuli act on the respective target organs to reestablish homeostasis (Fig. 3-7). Endocrine systems typically consist of closed-loop feedback mechanisms; if hormones from organ A stimulate organ B, organ B secretes hormones that inhibit the secretion of hormones from organ A. The concerted actions of positive and negative hormonal influences thereby maintain homeostasis. An example of negative feedback regulation is the control of the secretion of adrenocorticotropic hormone (ACTH) by the anterior pituitary gland. An increased ACTH level stimulates the adrenal cortex to produce and secrete cortisol, which suppresses further pituitary secretion of ACTH. These regulatory processes may include feedback loops in which nonhormonal substances controlled by the target organs regulate hormone secretion. For example, an increase in the concentration of plasma electrolytes as a Environment Sensory inputs

Brain

Bioaminergic or peptidergic neurons Hypothalamus –

Releasing factors –

Inhibiting factors

+

+ or –

–

Pituitary

– Target organ hormones

Tropic hormones +

Target organ

Figure 3-7 Regulatory feedback loops of the hypothalamic-pituitary–target organ axis. As combinations of stimulatory and inhibitory factors, hormones often act in concert to maintain homeostatic balance in the presence of physiologic or pathophysiologic perturbations. The concerted actions of hormones typically establish closed feedback loops by stimulatory and inhibitory effects coupled to maintain homeostasis.

Genetic Control of Peptide Hormone Formation   37

consequence of dehydration stimulates the release of arginine vasopressin (antidiuretic hormone) in the neural lobe of the pituitary gland, and vasopressin in turn acts on the kidney to increase reabsorption of water from the renal tubule, thereby readjusting serum electrolyte concentrations toward normal levels. In many instances, endocrine regulation is complex and involves the responses of several endocrine glands and their respective target organs. After a meal, the release of a dozen or more hormones is triggered as a result of gastric distention, variations in the pH of the contents of the stomach and duodenum, and increased concentrations of glucose, fatty acids, and amino acids in the blood. The rise in plasma glucose and amino acid levels stimulates the release of insulin and the incretin hormones glucagon-like peptide 1 and glucose-dependent insulinotropic peptide and suppresses the release of glucagon from the pancreas. Both effects promote the net uptake of glucose by the liver: insulin increases cellular transport and uptake of glucose, and the lower blood levels of glucagon decrease the outflow of glucose because of diminished rates of glycogenolysis and gluconeogenesis.

STRUCTURE OF A GENE ENCODING A POLYPEPTIDE HORMONE Structural analyses of gene sequences have resulted in at least three major discoveries that are important for understanding the expression of peptide-encoding genes. First, sequences of almost all of the known biologically active hormonal peptides are contained within larger precursors that often encode other peptides, many of which are of unknown biologic activity. Second, the transcribed regions of genes (called exons) are interrupted by sequences (called introns) that are transcribed but subsequently cleaved from the initial RNA transcripts during their nuclear processing and assembly into specific mRNAs. Third, specific

regulatory sequences reside in the regions of DNA flanking the structural genes and within introns, and these DNA sequences constitute specific targets for the interactions of DNA-binding proteins that determine the level of expression of the gene. The DNA of higher organisms is wound around proteins, forming tightly and regularly packed chromosomal structures called nucleosomes.33,34 Nucleosomes are composed of four or five different histone subunits that form a core structure about which approximately 140 base pairs of genomic DNA are wound. The nucleosomes are arranged similarly to beads on a string, and coils of nucleosomes form the fundamental organizational units of the eukaryotic chromosome. The nucleosomal structure serves several purposes. For example, nucleosomes enable the large amount of DNA (approximately 2 × 109 base pairs) included in the genome to be compacted into a small volume. Nucleosomes are involved in the replication of DNA and gene transcription. In addition to histones, other proteins are associated with DNA, and the complex nucleoprotein structure provides specific recognition sites for regulatory proteins and enzymes involved in DNA replication, rearrangements of DNA segments, and gene expression. The processes of acetylation, deacetylation, methylation, and demethylation of histones that are enriched in chromatin are implicated in the regulation of gene transcription (see “Epigenetic Inheritance of Phenotypic Traits”). The histones of open-configured chromatin (i.e., euchromatin) are heavily acetylated and methylated; this loosens their association with DNA and allows the access of transcription factors to the promoter regions of expressed genes. Conversely, the histones of closed chromatin (i.e., heterochromatin) are underacetylated and undermethylated, adhere tightly to DNA, and prevent access of transcription factors to the promoters of transcriptionally silent genes. A typical protein-encoding gene consists of two functional units (Fig. 3-8). One is a transcriptional region, and the other is a promoter or regulatory region.

DNA-binding proteins

Transcription initiation site 5´

TSS

TSE

Tissue-specific silencers/enhancers

MRE Metabolic response element

CP (TATA-box)

Exon

ATG Basal constitutive promoter

Intron

Exon

3´

TGA Transcription unit

Promoter region Figure 3-8  Structure of a consensus gene encoding a prototypical polypeptide hormone. A consensus gene typically consists of a promoter region and a transcription unit. The transcription unit is the region of DNA composed of exons and introns that is transcribed into a messenger ribonucleic acid (mRNA) precursor. Transcription begins at the cap site sequence in DNA and extends several hundred bases beyond the poly(A) addition site in the 3′ region. During post-transcriptional processing of the RNA precursor, the 5′ end of mRNA is capped by the addition of methylguanosine residues. The transcript is then cleaved at the poly(A) addition site approximately 20 bases 3′ to the AATAAA signal sequence, and the poly(A) tract is added to the 3′ end of the RNA. Introns are cleaved from the RNA precursor, and exons are joined together. Dinucleotides GT and AG are invariably found at the 5′ and 3′ ends of introns. Translation of mRNA starts with the codon ATG for methionine. Translation is terminated when the polyribosome reaches the stop codon TGA, TAA, or TAG. The promoter region of the gene located 5′ to the cap site contains numerous short regulatory DNA sequences that are targets for interactions with specific DNA-binding proteins. These sequences consist of the basal constitutive promoter (TATA box), metabolic response elements that modulate transcription (e.g., in response to cyclic adenosine monophosphate [cAMP], steroid hormone receptors, or thyroid hormone receptors), and tissue-specific enhancers and silencers that respectively permit or prevent transcription of the gene. The enhancer and silencer elements direct expression of specific subsets of genes to cells of a given phenotype. Whether a gene is or is not expressed in a particular cellular phenotype depends on complex interactions of the various DNA-binding proteins among themselves and, most importantly, with the TATA box proteins of the basal constitutive promoter.

38   Genetic Control of Peptide Hormone Formation

Transcriptional Regions The transcriptional unit is the segment of the gene that is transcribed into an mRNA precursor. The sequences corresponding to the mature mRNA consist of the exon sequences that are spliced from the primary transcript during cotranscriptional and post-transcriptional processing of the precursor RNA. These exons contain the code for the mRNA sequence that is translated into protein and for untranslated sequences at the 5′ and 3′ flanking regions. The 5′ sequence typically begins with a methylated guanine residue known as the cap site. The 3′ untranslated region contains within it a short sequence, AATAAA, that signals the site of cleavage of the 3′ end of the RNA and the addition of a poly(A) tract of 100 to 200 nucleotides located approximately 20 bases from the AATAAA sequence. Although the functions of these modifications of the ends of mRNAs are not completely understood, they appear to provide signals for leaving the nucleus; to enhance stability, perhaps by providing resistance to degradation by exonucleases; and to stimulate initiation of mRNA translation. The protein-coding sequence of the mRNA begins with the codon AUG for methionine and ends with the codon immediately preceding one of the three nonsense, or stop, codons: UGA, UAA, and UAG. The nature of the enzymatic splicing mechanisms that result in excision of intron-coded sequences and rejoining of exon-coded sequences is incompletely understood. Helpful interpretations of the splicing processes have been provided in recent reviews.35,36 Short “consensus” sequences of nucleotides reside at the splice junctions; for example, the bases GT and AG at the 5′ and 3′ ends of the introns, respectively, are invariant, and a polypyrimidine stretch is found near the AG.37 Splicing involves a series of cleavage and ligation steps that remove the introns as a lariat structure, with its 5′ end ligated near the 3′ end of the introns, and ligate the two adjacent exons together. The spliceosome, an elaborate mechanism consisting of five small nuclear RNAs (snRNAs) and roughly 50 proteins, directs these steps, guided by base pairing between three of the snRNAs and the mRNA precursor.

Regulatory Regions The molecular mechanisms involved in regulating the expression of genes that encode polypeptides are becoming understood in some detail. Some experiments have deleted certain 5′ sequence segments that reside upstream from structural genes and then analyzed the expression of those genes after their introduction into cell lines. These regulatory sequences, called promoter and enhancer regions, consist of short polynucleotide sequences (see Fig. 3-8). They can be divided into at least four groups with respect to their functions and distance from the transcriptional initiation site. First, the sequence TATAA (TATA box or GoldbergHogness box) is usually present in the more proximal promoter within 25 to 30 nucleotides upstream from the point of transcriptional initiation. The TATA sequence is required to ensure the accuracy of initiation of transcription at a particular site. The TATA box directs the binding of a complex of several proteins, including RNA polymerase II. The proteins, referred to as TATA box transcription factors, number six or more basal factors (IIA, IIB, IID, IIE, IIF, IIH); along with RNA polymerase II, they form the general or basal transcriptional machinery required for the initiation of RNA synthesis.38-40 The other three groups of regulatory sequences are the tissue-specific silencers (TSSs), which function by binding

repressor proteins; the tissue-specific enhancers (TSEs), which are activated by the binding of transcriptional activator proteins; and the metabolic response elements (MREs), which are regulated by the binding of specialized proteins whose transcriptional (repressor or activator) activities are determined by metabolic signaling that often involves changes in their phosphorylation status.

Introns and Exons Genes encoding proteins and ribosomal RNAs in eukaryotes are interrupted by intervening DNA sequences (i.e., introns) that separate them into coding blocks (i.e., exons).35,36,41 In bacterial genes, the nucleotide sequences of the chromosomal genes match precisely the corresponding sequences in the mRNAs. Interruption of the continuity of genetic information appears to be unique to nucleated cells. The reasons for such interruptions are not completely understood, but introns appear to separate exons into functional domains with respect to the proteins that they encode. An example is the gene for proglucagon, a precursor of glucagon in which five introns separate six exons, three of which encode glucagon and the two glucagonrelated peptides contained within the precursor (Fig. 3-9).42 A second example is the GH gene, which is divided into five exons by four introns that separate the promoter region of the gene from the protein-coding region and divide the latter into three partly homologous repeated segments, two of which code for the growth-promoting activity of the hormone and the third for its carbohydrate metabolic functions.43 As a rule, the genes for the precursors of hormones and regulatory peptides contain introns that are located at or about the region where signal peptides join the apoproteins or prohormones and thus separate the signal sequences from the components of the precursor that are exported from the cell as hormones or peptides. There are exceptions to the “one exon, one function” theory in mammalian cells. The genes of several precursors of peptide hormones are not interrupted by introns in a manner that corresponds to separation of the functional components of the precursor. For example, the precursor pro-opiomelanocortin (POMC) is cleaved during posttranslational processing to produce the peptides ACTH, α-melanocyte–stimulating hormone, and β-endorphin, but the protein-coding region of the POMC gene is devoid of introns. Likewise, no introns interrupt the protein-coding region of the gene for the proenkephalin precursor, which contains seven copies of the enkephalin sequences. It is possible that introns separated each of these coding domains in the past and were lost during the course of evolution. A precedent for the selective loss of introns appears to be exemplified by the rat insulin genes. The rat genome harbors two nonallelic insulin genes, one containing two introns and the other containing a single intron. The most likely explanation is that an ancestral gene containing two introns was transcribed into RNA and spliced, after which that RNA was copied back into DNA by a cellular reverse transcriptase and inserted into the genome at a new site.

REGULATION OF GENE EXPRESSION Regulation of expression of genes encoding polypeptide hormones can take place at one or more levels in the pathway of hormone biosynthesis44-46 (Fig. 3-10): DNA synthesis (cell growth and division), transcription,

Genetic Control of Peptide Hormone Formation   39 E1

IA

E2

E3

IB

IC

E4

E5

ID

IE

E6

Gene 3´

5´

M

Q H

K R

K K R R

R R R R

K K

mRNA 5´

3´ UN-TX S

N

Gluc

GLP-I IP-I

GLP-II

UN-TX

IP-II

Figure 3-9 The pancreatic glucagon gene and its encoded messenger RNA (mRNA): complementary DNA. In the glucagon gene, exons precisely encode separate functional domains. The gene consists of six exons (E1 through E6) and five introns (1A through 1E). The mRNA encoding pre-proglucagon, the protein precursor of glucagon, consists of 10 specific regions: from left to right, a 5′ untranslated sequence (UN-TX, open), a signal sequence (S, stippled), an amino-terminal extension sequence (N, hatched), glucagon (Gluc, shaded), a first intervening peptide (IP-I, hatched), a first glucagon-like peptide (GLP-I, shaded), a second intervening peptide (IP-II, hatched), a second glucagon-like peptide (GLP-II, shaded), a dilysyl dipeptide (hatched) after the GLP-II sequence, and an untranslated region (UN-TX, open). Exons from left to right encode the 5′ untranslated region, signal sequence, glucagon, GLP-I, GLP-II, and 3′ untranslated sequence. Letters shown above the mRNA denote amino acids located at positions in the pre-proglucagon molecule that are cleaved during cellular processing of the precursor. The amino acid methionine (M) marks the initiation of translation of mRNA into pre-proglucagon. H, histidine; K, lysine; Q, glutamine; R, arginine.

post-transcriptional processing of mRNA, translation, or post-translational processing. In different endocrine cells, one or more levels may serve as specific control points for regulation of the production of a hormone (see “Biologic Diversification”).

Levels of Gene Control Newly synthesized prolactin transcripts are formed within minutes after exposure of a prolactin-secreting cell line to TRH.47 Cortisol has been demonstrated to stimulate GH synthesis in somatotropic cell lines and in pituitary slices through increases in rates of gene transcription and enhancement of the stability of mRNA.48,49 The time required for cortisol to enhance transcription of the GH gene is 1 to 2 hours, which is considerably longer than the time required for the action of TRH on prolactin gene transcription. Regulation of proinsulin biosynthesis appears to take place primarily at the level of translation.50,51 Within minutes after the plasma glucose level is raised, the rate of proinsulin biosynthesis increases 5- to 10-fold. Glucose acts

directly or indirectly to enhance the efficiency of initiation of translation of proinsulin mRNA.52 Rapid metabolic regulation at the level of posttranscriptional processing of mRNA precursors has not been clearly established. However, alternative exon splicing plays a major role in the regulation of the formation of mRNAs during development (see “Biologic Diversification”). For example, the primary RNA transcripts derived from the calcitonin gene are alternatively spliced to provide two or more tissue-specific mRNAs that encode chimeric protein precursors with both common and different amino acid sequences, indicating that regulation takes place at the level of processing of the calcitonin gene transcripts. In many instances, the level of gene expression under regulatory control is optimal for meeting the secretory and biosynthetic demands of the endocrine organ. For example, after a meal, there is an immediate requirement for the release of large amounts of insulin. This release depletes insulin stores of the pancreatic beta cells within a few minutes, but mechanisms that increase the translational

Cell Nucleus

Circulation Ca2+

Ca2+

1. DNA synthesis Figure 3-10  Potential control points in an endocrine cell for regulation of gene expression during hormone production. Specific effector substances bind to plasma membrane receptors (peptide effectors) or to cytosolic or nuclear receptors (steroids), which leads to initiation of a series of events that couple the effector signal with gene expression. Peptide effector–receptor complex interactions act initially through activation of adenylate cyclase (AC) coupled with a guanosine triphosphate–binding protein (G). Coupling factors and substances such as glucose, cyclic adenosine monophosphate, and cations activate protein kinases, resulting in a series of phosphorylations of macromolecules. Specific effectors for various endocrine cells appear to act at one or more of the indicated five levels of gene expression (see text for details), with the possible exception of post-translational processing of prohormones, for which no definite examples of metabolic regulation have yet been found.

DNA 2. Transcription PremRNA 3. Post-transcriptional processing

Receptor AC Effector G

mRNA ?

4. Translation Pre-proHormone

Coupling factors and messengers

ProHormone

5. Post-translational processing

Hormone Stores

Hormone

40   Genetic Control of Peptide Hormone Formation efficiency of preformed proinsulin mRNA rapidly provide additional hormone.

Tissue-Specific Gene Expression Differentiated cells have a remarkable capacity for selective expression of specific genes. In one cell type, a single gene may account for a large fraction of the total gene expression, and in another cell type, the same gene may be expressed at undetectable levels. When a gene can be expressed in a particular cell type, the associated chromatin is loosely arranged; when the same gene is never expressed in a particular cell type, the chromatin organization is more compact. DNA within the chromatin of expressed genes is more susceptible to cleavage by deoxyribonuclease than is the DNA in tissues in which the genes are quiescent.53-55 Chromatin looseness may facilitate access of RNA polymerase to the gene for purposes of transcription. Inactive genes appear to have a higher content of methylated cytosine residues than the same genes in tissues in which they are expressed.56,57 Determinants for the tissue-specific transcriptional expression of genes exist in control sequences that usually reside within 1000 base pairs of the 5′-flanking region of the transcriptional sequence. Enhancer sequences in animal cell genes were first described for immunoglobulin genes, a finding that extended the earlier observations of enhancer control elements in viral genomes.58 Historically, the first clear demonstrations of these elements directing transcription to cells of distinct phenotypes came from studies of the comparative expression of two model genes, insulin and chymotrypsin, in the endocrine and exocrine pancreas, respectively.59 The restricted expression of genes in a cell-specific manner is determined by the assembly of specific combinations of DNA-binding proteins on a predetermined array of control elements of the promoter regions of genes to create a transcriptionally active complex of proteins that includes the components of the general or basal transcriptional apparatus.

Epigenetic Inheritance of Phenotypic Traits Darwinian selection is the generally recognized mechanism by which changes in the nucleotide sequences of genes result in patterns of gene expression that allow the resultant phenotype to better survive in a changing environment. These changes in gene sequences that lead to prosurvival phenotypes occur gradually, over hundreds of generations spanning thousands of years of evolution. However, genomic changes in gene expression in response to environmental changes may occur during development within a single generation by epigenetic mechanisms, independently of any nucleotide sequence changes in the DNA.60 Epigenetics refers to the inheritable phenotypic changes that do not involve alterations in DNA, a nongenetic memory of gene expression functions imparted by environmental cues. Epigenetics defines a nongenetic memory of function that is transmitted from cellular generation to generation.61 The mechanism of epigenetic memory is beginning to be understood. It involves chromatin modifications—both modifications of cytosines in DNA and post-translational modifications of proteins that collectively define the structure of chromatin. Broadly speaking, chromatin exists in two configurations that determine whether the involved genes are expressed (active) or are repressed (silent). The two configurations are densely packed heterochromatin, in which gene transcription is silent, and euchromatin, in which DNA and

proteins are loosely gathered, allowing access of transcription factors to DNA promoters of genes and active transcription. The protein components of chromatin consist of histones and nonhistone proteins. The key to epigenetic memory, the collective state of expressed or repressed genes in the genome, lies with the methylation of DNA and the post-translational modifications of histones, predominantly by acetylation and methylation. These modifications are interpreted by an elaborate series of protein complexes that change the structure of chromatin and the activity of genes.62 The methylation of cytosines in DNA in the configuration of CpG sequences (CpG islands), represses gene transcription by impairing productive interactions of transcription factors with DNA promoter sequences. Histones can be grouped into four major classes: H2A, H2B, H3, and H4. Histones carry a strong positive charge because of their large content of the amino acids lysine and arginine. They bind firmly to negatively charged nucleic acids and phosphates of DNA, form heterochromatin, and thereby repress gene expression. The covalently modified histones bind to specific effector proteins that recognize them. Attachment of hydrophobic side chains to the positively charged lysines (and arginines) of histones reduces the positive charge and loosens binding to DNA. Such side chain modifications include acetylation, methylation, ADP-ribosylation, sumoylation, phosphorylation, and ubiquitination. Acetylation and methylation are predominant mechanisms of histone modification. DNA methylation is regulated by a family of cytosinedirected methylases and demethylases. Methylation and acetylation of histones is mediated by methyl and acetyl transferases, demethylases, and deacetylases. The effects of DNA methylation and histone acetylation and methylation on gene expression are complex. DNA methylation usually is associated with gene silencing, histone acetylation with gene activation, and histone methylation with either silencing or activation. DNA methylation and histone methylation may bidirectionally regulate each other. For example, DNA methylation induces methylation of lysine 8 of histone 3 (H3K9), and, conversely, methylation of H3K9 enhances DNA methylation. Both DNA methylation and H3K9 methylation repress transcription, so they reinforce each other in silencing genes. These changes in chromatin modifications are durable; they can be passed from parent to daughter cells during cell division and, to some extent, can remain during meiosis. In this way, patterns of gene expression imparted by environmental influences are preserved during cell growth and reproduction. An example of environmentally induced epigenetic modification of gene expression that is reflected in a modified disease phenotype is the remarkable reprogramming of the genome that takes place in utero in rat embryos subjected to nutrient deprivation by partial ligation of the uterine artery during the last 3 days of gestation (days 19 to 22).63 The late prenatal, nutrient-deprived pups go on to progressively develop obesity, insulin resistance, severe diabetes, and premature death. Biochemical studies of the affected nutrient-deprived progeny show that key genes involved in insulin secretion and sensitivity are repressed by the transient nutrient deprivation. For example, mRNA levels for the transcription factor PDX1, which is critical for pancreatic beta cell growth and insulin production, is reduced as early as 24 hours after growth retardation, and histone H3 in the chromatin of the PDX1 promoter is hypomethylated on lysine 4 and hypoacetylated on lysine 9, defining a heterochromatin configuration and gene silencing. The growth and functions of beta cells are

Genetic Control of Peptide Hormone Formation   41

curtailed by the nutrient deprivation to produce less insulin, because less insulin is needed and excessive insulin would be deleterious to the embryo. Likewise, the histones associated with the promoters of the liver genes PPARGC1A (formerly called PGC1A), which is involved in gluconeogenesis, and CPT1A, which is involved in fatty acid oxidation, are hyperacetylated, resulting in activation of these genes and loss of insulin regulation of glucose use and oxidation (i.e., insulin resistance). The mechanisms by which acetylases, methylases, deacetylases, and demethylases are regulated remain unknown. One possibility is that intermediary metabolites formed in response to nutrient deprivation, such as acetyl coenzyme A, nicotinamide adenine dinucleotide, and S-adenosyl methionine, may modulate the relative activities of these important enzymes.

Transcription Factors in Developmental Organogenesis of Endocrine Systems Certain families of transcription factors are critical for organogenesis and the development of the body plan. Among these are the homeodomain proteins64 and the nuclear receptor proteins.65-67 The family of homeotic selector, or homeodomain, proteins is highly conserved throughout the animal kingdom from flies to humans. The orchestrated spatial and temporal expression of these proteins and the target genes that they activate determine the orderly development of the body plan of specific tissues, limbs, and organs. Similarly, the actions of families of nuclear receptors (e.g., steroid hormones, thyroid hormones, retinoic acid) are critical for normal development. Inactivating mutations in the genes encoding these essential transcription factors predictably result in loss or impairment of the development of the specific target organ. This section describes three examples of impaired organogenesis attributable to mutations in essential transcription factors: partial anterior pituitary agenesis (POU1F1), pancreatic agenesis (PDX1), and adrenal and gonadal agenesis (SF1 and NROB1/DAX1).

Partial Pituitary Agenesis The transcription factor POU1F1, formerly called Pit-1, is a member of the POU family of homeodomain proteins.68 POU1F1 is a key transcriptional activator of the promoters of the genes that encode GH, prolactin, and thyroidstimulating hormone-β, which are produced in the anterior pituitary somatotrophs, lactotrophs, and thyrotrophs, respectively. POU1F1 is also the major enhancer activating factor for the promoter of the growth hormone–releasing factor receptor gene.69 Mutations in POU1F1 that impair its DNA-binding and transcriptional activation functions are responsible for the phenotype of the Jackson and Snell dwarf mice.68 Mutations in the POU1F1 gene have been found in patients with combined pituitary hormone deficiency, a condition in which there is no production of GH, prolactin, or thyroid-stimulating hormone, which results in growth impairment and mental deficiency.70 In these individuals, hormone production by the other two of the five cell types of the anterior pituitary gland (including production of ACTH and the gonadotropins luteinizing hormone and follicle-stimulating hormone) remains unaffected.70 The mutated POU1F1 can bind to its cognate DNA control elements but is defective in trans-activating gene transcription. Furthermore, it acts as a dominant negative inhibitor of the actions of POU1F1 on the unaffected allele.

Pancreatic Agenesis The homeodomain protein called pancreas duodenum homeobox 1 or PDX1 (previously referred to as STF1, IDX1, or IPF1) appears to be responsible for the development and growth of the pancreas. Targeted disruption of the Pdx1 gene in mice resulted in a phenotype of pancreatic agenesis.71 A child born without a pancreas was shown to be homozygous for inactivating mutations in the PDX1 gene.72 Parents and ancestors who are heterozygous for the affected allele have a high incidence of type 2 diabetes mellitus, suggesting that a decrease in gene dosage of PDX1 may predispose the individual to development of diabetes. The possibility that a mutated PDX1 allele may be one of several “diabetes genes” is supported by the observation that PDX1 and the helix-loop-helix transcription factors E47 (TCF3) and β2 (TFB2M) appear to be key upregulators of the transcription of the insulin gene.73

Agenesis of the Adrenal Gland and Gonads Two nuclear receptor transcription factors have been identified as critical for the development of the adrenal gland, the gonads, pituitary gonadotrophs, and the ventral medial hypothalamus. These nuclear receptors are SF1 (steroidogenic factor 1)74 and NROB1 (also called DAX1, for dosagesensitive sex reversal, adrenal hypoplasia congenita, X chromosome).75 SF1 binds to half-sites of estrogen response elements that bind estrogen receptors in the promoters of genes. DAX1 binds to retinoic acid receptor (RAR) binding sites in promoters and inhibits RAR actions. Targeted disruption of SF1 in mice results in a phenotype of adrenal and gonadal agenesis. Pituitary gonadotrophs are absent, and the ventral medial hypothalamus is severely underdeveloped.76,77 Adrenal hypoplasia congenita is an X-linked developmental disorder of the human adrenal gland that is lethal if untreated. The gene responsible for adrenal hypoplasia congenita was identified by positional cloning and found to encode NROB1/DAX1, a member of the nuclear receptor proteins related to RAR.75 Several inactivating mutations identified in the gene result in the syndrome of adrenal hypoplasia congenita and hypogonadotropic hypogonadism. Genetically defined and transmitted defects in the genes encoding the transcription factors SF1 and DAX1 result in profound arrest in the development of the target organs regulated by the hypothalamic-pituitary-adrenal axis involved in steroidogenesis: the adrenal gland (e.g., glucocorticoids, mineralocorticoids) and the gonads (e.g., estrogens, androgens).

Coupling of Effector Action to Cellular Response Another mode of gene control consists of the induction and suppression of genes that are normally expressed in a specific tissue. These processes are at work in the minuteto-minute and day-to-day regulation of rates of production of the specific proteins produced by the cells (e.g., production of polypeptide hormones in response to extracellular stimuli). At least two classes of signaling pathways—protein phosphorylation and activation of steroid hormone receptors by hormone binding—appear to be involved in the physiologic regulation of hormone gene expression. These two pathways mediate the actions of peptide and steroid hormones, respectively. Peptide ligands bind to receptor complexes on the plasma membrane, which results in enzyme activation, mobilization of calcium,

42   Genetic Control of Peptide Hormone Formation formation of phosphorylated nucleotide intermediates, activation of protein kinases, and phosphorylation of specific regulatory proteins such as transcription factors (see Chapter 5).77,79 Because of their hydrophobic composition, steroidal compounds readily diffuse through the plasma membrane, bind to specific receptor proteins, and interact with other macromolecules in the nucleus, including specific domains on the chromatin located in and around the gene that is activated (see Chapter 4).65-67 Calcium and phosphorylated nucleotides such as cyclic adenosine monophosphate (cAMP), adenosine triphosphate, and guanosine triphosphate appear to have important functions in secretory processes. In particular, fluxes of calcium from the extracellular fluid into the cell and from intracellular organelles (e.g., ER) into the cytosol are closely coupled to secretion.80,81 The many cellular signaling pathways that involve protein phosphorylations are complex. They typically consist of sequential phosphorylations and dephosphorylations of molecules, referred to as protein kinase cascades or phosphatase cascades.82 These cascades are initiated by hormones, sensor molecules known as ligands that bind to and activate receptors located on the surface of cells, resulting in the generation of small second-messenger molecules such as cAMP, diacylglycerol, or calcium ions. These second messengers activate protein kinases that phosphorylate and thereby activate key target proteins (Fig. 3-11). The final step in the signaling pathways is the phosphorylation and activation of important transcription factors resulting in gene expression or repression. The identities of some phosphoproteins have been elucidated. A specific group of transcription factors, DNAbinding proteins, interacts with cAMP-responsive and phorbol ester–responsive DNA elements to stimulate gene transcription mediated by the cAMP–protein kinase A, diacylglycerol–protein kinase C, and calcium-calmodulin signal transduction pathways (see Fig. 3-11). These proteins are encoded by a complex family of genes and bind to the

DNA elements in the form of heterodimers or homodimers through a coiled-coil helical structure known as a leucine zipper motif.83 Phosphorylation of these proteins modulates dimerization, DNA recognition and binding, and transcriptional trans-activation activities. Phosphorylation of the protein substrates may change their conformations and activate the proteins, which then interact with coactivator proteins such as the cAMP response element–binding (CREB) protein and the protein components of the basal transcriptional machinery, allowing RNA polymerase to initiate gene transcription.84 Second messengers activate serine/threonine kinases, which phosphorylate serine or threonine residues (or both) on proteins, whereas the receptor kinases are tyrosine-specific kinases that phosphorylate tyrosine residues.82,85 Examples of receptor tyrosine kinases are growth factor receptors such as those for insulin, insulin-like growth factor (IGF), epidermal growth factor, and plateletderived growth factor. Receptors in the cytokine receptor family, which include leptin, GH, and prolactin, activate associated tyrosine kinases in a variation on the theme. The different types of signal transduction pathways are described as more or less distinct pathways for semantic purposes. In reality, there is considerable cross-talk among the different pathways that occur developmentally and in cell type–specific settings. An active area of research in endocrine systems is attempting to understand these complex interactions among different signal transduction pathways. Although the growth factor and cytokine receptors are similar in some respects, they differ in other respects. For example, growth factor receptor tyrosine kinases activate transcription factors through cascades that involve both tyrosine phosphorylation and serine/ threonine kinases such as mitogen-activated protein kinases, whereas the Janus kinases (JAKs), activated by cytokine receptors, directly tyrosine phosphorylate the signal transducer and activator of transcription (STAT) factors.85,86

First sensor (receptor)

Second mediator (small diffusible)

Third effector (kinase)

Fourth transcription factor (DNA-binding protein)

H1

R1

DAG

PKC

Phosphorylation

H2

R2

cAMP

PKA

H3

R3

Ca2+

CaMK

Messenger: Function: Molecule:

P Transcription Nucleus P

Cytoplasm

Figure 3-11 Three cell surface receptor–coupled signal transduction pathways involved in the activation of a superfamily of nuclear transcription factors. Peptide hormone molecules (H1, H2, and H3) interact with sensor receptors (R1, R2, and R3) coupled to the diacylglycerol (DAG)–protein kinase C (PKC), cyclic adenosine monophosphate (cAMP)–protein kinase A (PKA), and calcium-calmodulin pathways in which small, diffusible second-messenger molecules are generated (DAG, cAMP, and Ca2+, respectively). The third messengers, or effector protein kinases, are generated and phosphorylate transcription factors such as members of the CREB/ATF and JUN/AP-1 families of DNA-binding proteins to modulate DNA-binding affinities or transcriptional activation, or both. The various proteins bind as dimers determined by a poorly understood code that is not promiscuous (i.e., only certain homodimer or heterodimer combinations are permissible). AP-1, activator protein 1; ATF, activating transcription factor; CaMK, calcium/calmodulin-dependent protein kinase; CREB, cAMP response element–binding protein.

Genetic Control of Peptide Hormone Formation   43

BIOLOGIC DIVERSIFICATION In addition to providing control points for the regulation of gene expression, the various steps involved in transfer of information encoded in the DNA of the gene to the final bioactive protein are a means for diversification of information stored in the gene (Fig. 3-12). Five steps in gene expression can be arbitrarily described: gene duplication and copy number, transcription, post-transcriptional RNA processing, translation, and post-translational processing.

Gene Duplications At the level of DNA, diversification of genetic information comes about by way of gene duplication and amplification. Many of the polypeptide hormones are derived from families of multiple, structurally related genes. Examples include the growth hormone family (GH, prolactin, and placental lactogen), the glucagon family (glucagon, vasoactive intestinal peptide, secretin, gastric inhibitory peptide, and growth hormone–releasing hormone), and the glycoprotein hormone family (thyrotropin, luteinizing hormone, follicle-stimulating hormone, and chorionic gonado­ tropin). A remarkable example of diversification at the level of gene amplifications is the extraordinarily large number of genes encoding the pheromone and odorant receptors.87 As many as 1000 of these receptor genes may exist in mouse and rat genomes, each receptive to a particular odorant ligand. Over the course of evolution, an ancestral gene encoding a prototypic polypeptide representative of each of these families was duplicated one or more times, and through mutation and selection, the progeny proteins of the ancestral gene assumed different biologic functions.

I

D I V E R S I F I C A T I O N

The exonic-intronic structural organization of the genomes of higher animals lends itself to gene recombination and RNA copying of genetic sequences with subsequent reintegration of DNA reverse-transcribed sequences back into the genome, resulting in rearrangement of transcriptional units and regulatory sequences.88,89

Transcription In addition to duplication of genes and their promoters, another way to create diversity in expression is at the level of gene transcription, by providing genes with alternative promoters90 and by using a large array of cis-regulatory elements in the promoters, governed by complex combinations of transcription factors.

Alternative Promoters Many of the genes encoding hormones and their receptors use more than one promoter during development or when expressed in different tissue types. Employment of alternative promoters results in the formation of multiple transcripts that are different at their 5′ ends (Fig. 3-13). It is presumed that some genes have multiple promoters because they provide flexibility in the control of expression of the genes. For example, in some cases, expression of genes in more than one tissue or developmental stage requires distinct combinations of tissue-specific transcription factors. This flexibility enables genes in different cell types to respond to the same signal transduction pathways or genes in the same cell type to respond to different signal transduction pathways. A single promoter may not be adequate to respond to a complex array of transcription factors and a changing environment of cellular signals. The organization of alternative promoters in genes manifests in several patterns within exons or introns in the 5′

II

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3) Varying transcriptional stops and alternative splicing of transcripts 4) Regulation of translation 5) Alternative processing of polyproteins

Biologic action Figure 3-12 Levels in expression of genetic information at which diversification of information encoded in a gene may take place. The three major levels of genetic diversification are (1) gene duplication, a process that occurs in terms of evolutionary time; (2) variation in the processing of ribonucleic acid (RNA) precursors, which results in formation of two or more messenger RNAs (mRNAs) by way of alternative pathways of splicing of transcript (see Figs. 3-13 and 3-14); and (3) use of alternative patterns in processing of protein biosynthetic precursors (polyproteins or prohormones). These mechanisms provide a means for diversification of gene expression at levels of deoxyribonucleic acid (DNA), RNA, or protein. One or a combination of these processes leads to formation of the final biologically active peptide or hormone. Loops depicted in transcripts denote introns; in protein structures, the stippled, shaded, and open areas denote exons. A, B, and C represent splicing intermediates that lead to two distinct transcripts (1, 2) from one precursor. E1-4 indicate portions of polyproteins cleaved post-translationally as indicated. I, II, and III indicate multiple transcribed genes following gene duplication.

44   Genetic Control of Peptide Hormone Formation Exonic noncoding

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Figure 3-13  Use of alternative promoters in the expression of genes as a means to generate biologic diversification of gene expression. The use of alternative promoters allows a gene to be expressed in a variety of contexts that alter the properties of the messenger ribonucleic acid (mRNA) that is expressed. Alternative promoter use may render the mRNA more or less stable, affect translational efficiencies, or switch the translation of one protein isoform to another. The use of alternative promoters in genes characteristically occurs during development or, after development is completed, to designate tissue-specific patterns of expression of the gene. Exons are shown as boxes whose protein-coding regions are shaded. Introns are designated by horizontal lines. Dashed lines indicate introns that are spliced out. (Adapted from Ayoubi TAY,Van De Ven WJM. Regulation of gene expression by alternative promoters. FASEB J. 1996;10:453-460.)

noncoding sequence or the coding sequence (see Fig. 3-13). The most common occurrence of alternative promoters is within the 5′ noncoding or leader exons. Use of different promoters in the 5′ untranslated region of a gene, often accompanied by alternative exon splicing, results in the formation of mRNAs with different 5′ sequences. The alternative use of promoters in 5′ leader exons can affect gene expression and generate diversity in several ways, including developmental stage-specific and temporal expression of genes, tissue-type specificity of expression, levels of expression, responsiveness of gene expression to specific

metabolic signals conveyed through signal transduction pathways, stability of the mRNAs, efficiencies of translation, and structures of the N-termini of proteins encoded by the genes.90 Examples of genes that use alternative 5′ leader promoters during development are those encoding IGF1, IGF2, the RARs, and glucokinase, all of which are regulated by multiple promoters that are active in a variety of embryonic and adult tissues and are subject to developmental and tissue-specific regulation.90 During fetal development, promoters P2, P3, and P4 of the IGF2 gene are active in the liver. These promoters are shut off after birth, at which time the P1 promoter is activated. The P1 and P2 promoters of the IGF1 gene are differentially responsive to GH; P2 expressed in liver is responsive to GH, whereas P1 expressed in muscle is not. The RAR exists in alpha, beta, and gamma isoforms (RARA, RARB, and RARG), encoded by separate genes that give rise to at least 17 different mRNAs generated by a combination of multiple promoters and alternative splicing.91 The RAR isoforms have different specificities for retinoic acid–responsive promoters, different affinities for ligand isoforms, and different trans-activating capabilities. The various RAR isoforms are expressed at different times in different tissues during development. It has been proposed that the multiple RAR isoforms provide a means of achieving a diverse set of cellular responses to a single ligand, retinoic acid.91 Glucokinase is an example of the alternative use of 5′ leader promoters that have different levels of metabolic responsiveness.92 Expression of glucokinase in pancreatic beta cells and some other neuroendocrine cells uses an upstream promoter (1β), whereas in liver, a promoter (IL) located 26 kilobases downstream of the 1β promoter is used exclusively. In beta cells, expression of the glucokinase gene is apparently not responsive to hormones. In liver expression mediated by the IL promoter, it is intensely upregulated by insulin and downregulated by glucagon. The α-amylase gene provides an example in which two alternative promoters in the 5′ noncoding exons expressed in two different tissues have dramatically different strengths of expression.90 A strong upstream promoter directs expression within the parotid gland, whereas weak expression is directed by an alternative downstream promoter in liver. Examples of the use of alternative promoters in the coding regions of genes include the progesterone receptor (PR) and the transcription factor, cAMP response element modulator (CREM). In both of these cases, different protein isoforms are produced that have markedly different functional activities. The genes encoding the chicken and human PRs express two isoforms of the receptor.93 Isoform A initiates translation at a methionine residue located 164 amino acids downstream from the methionine that initiates the translation of the longer form, isoform B. Analyses of the mechanisms responsible for the synthesis of the two isoforms revealed that two promoters exist in the human PR gene, one upstream of the 5′ leader exon and the other in the first protein-coding exon. The two isoforms of the human PR differ markedly in their abilities to trans-activate transcription from different progesterone-responsive elements (PREs). Both of the human PR isoforms equivalently activate a canonical PRE. Isoform B is much more efficient than A at activating the PRE in the mouse mammary tumor virus promoter, whereas isoform A, but not B, activates transcription from the ovalbumin promoter.93 Use of an alternative intronic promoter within the protein-coding sequence of a gene is exemplified by CREM.94 The CREM gene employs a constitutively active,

Genetic Control of Peptide Hormone Formation   45

unregulated promoter (P1) that encodes predominantly activator forms of CREM and an internal promoter (P2) located in the fourth intron that is regulated by cAMP signaling and encodes a repressor isoform known as inducible cAMP early response (ICER). The remarkable complexity of the alternative mechanisms of expression of the CREM and CREB genes is discussed later.

Exon skipping/switching ATG

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Diversity of Transcription Factors Diversity at the level of gene transcription can be created by the interplay of multiple transcription factors on multiple cis-regulatory sequences. The promoters of typical genes may contain 20 to 30 or more cis-acting control elements in the form of enhancers or silencers. These control elements may respond to ubiquitous transcription factors found in all cell types and to cell type–specific factors. Unique patterns of control of gene expression can be affected by several mechanisms acting in concert. The spacing, relative locations, and juxtapositioning of control elements with respect to each other and to the basal transcriptional machinery influence levels of expression. Transcription factors often act in the form of dimers or higher oligomers among factors of the same or different classes. A given transcription factor may act as either an activator or a repressor, according to the existing circumstances. The ambient concentrations of transcription factors within the nucleus, in conjunction with their relative DNA-binding affinities and trans-activation potencies, may determine the levels of expression of genes.

Post-transcriptional Processing: Alternative Exon Splicing Identification of the mosaic structure of transcriptional units encoding polypeptide hormones and other proteins that consist of exons and introns raised the possibility that the use of alternative pathways in RNA splicing could provide informationally distinct molecules. Different proteins could arise by inclusion or exclusion of specific exonic segments or by use of parts of introns in one mRNA as exons in another mRNA. Differences in the splice sites would result in expression of new translational reading frames. Alternative splicing uses two distinct mechanisms (Fig. 3-14). One is exon skipping, or switching in or out of exons. The other, known as intron slippage, includes use of part of an intron in an exon, splicing out of part of an exon along with the intron, and use of a “coding” intron. Both mechanisms can generate diversity in endocrine systems. Included among the genes encoding prohormones in which the pre-mRNAs are alternatively spliced by exon skipping or switching are those for procalcitonin/calcitonin gene–related peptide, prosubstance P/K, and the prokininogens. Alternative processing of the RNA transcribed from the calcitonin gene results in production of an mRNA in neural tissues that is distinct from that formed in the C cells of the thyroid gland.95 The thyroid mRNA encodes a precursor to calcitonin, whereas the mRNA in the neural tissues generates the neuropeptide, calcitonin gene–related peptide. Immunocytochemical analyses of the distribution of the peptide in brain and other tissues suggest functions for the peptide in perception of pain, ingestive behavior, and modulation of the autonomic and endocrine systems. Splicing of the RNA precursor that encodes substance P can take place in at least two ways.96 One splicing pattern results in the mRNA that encodes substance P and another peptide called substance K, in a common protein precursor. Other mRNAs are apparently spliced to exclude the coding sequence for substance K. An alternative RNA splicing pattern also occurs in the processing of transcripts arising

Intron slippage

Figure 3-14 Alternative exon splicing provides a means to generate biologic diversification of gene expression. Mechanisms of exon skipping or switching and intron slippage are frequently used in the alternative processing of precursor-messenger ribonucleic acids (mRNAs) to provide unique mRNAs and encoded proteins during development and in a tissue-specific pattern of expression in fully differentiated tissues or organs. Exons are shown as boxes with protein-coding regions shaded to designate origin of protein isoforms. Introns are depicted as horizontal lines. Dashed lines denote spliced-out introns.

from the gene encoding bradykinin.97 The high-molecularweight and low-molecular-weight kininogens are translated from mRNAs that differ in their alternative use of 3′-end exons encoding the C-termini of the prohormones—a situation similar to that found in the transcription of the calcitonin gene. Other examples of genetic diversification arise from programmed flexibility in the choice of splice acceptor sites within coding regions (i.e., intron slippage), which allows an array of coding sequences (exons) to be put together in a number of useful combinations. For example, the coding sequences of the GH, lutropin-choriogonadotropin,98 and leptin receptors99 can be brought together in two different ways, one to include and the other to exclude an exonic coding sequence that specifies the transmembranespanning domains of the polypeptide chains that anchor the receptors to the surface of cells. If mRNA splicing excludes the anchor’s peptide sequence, a secreted rather than a surface protein is produced.

Translation The process of translation provides a fourth level for creation of diversity in gene expression. The rate of translational initiation can be regulated, as discussed earlier; this is typified by the proinsulin and prohormone convertase mRNAs, in which translation is augmented by glucose and cAMP. Molecular diversity of translation is generated by the developmentally regulated use of alternative translation initiation (start) codons (i.e., methionine [AUG] codons). The mechanism of translation initiation involves assembly

46   Genetic Control of Peptide Hormone Formation Loose scanning AUG

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Figure 3-15 Alternative translational initiation sites are used to change the coding sequences of messenger ribonucleic acids to encode different protein isoforms. The two mechanisms shown involve loose scanning and reinitiation of translation (see text for details).

of the 40S ribosome subunit on the 5′ methyl guanosine cap of the mRNA.100 The ribosome subunit then scans 5′ to 3′ along the mRNA until it encounters an AUG sequence in a context of surrounding nucleotides favorable for initiation of protein synthesis. On encountering a favorable AUG, the subunit pauses and recruits the 60S subunit and a number of other essential translation initiation factors, allowing the polymerization of amino acids. The use of an alternative downstream start codon for translation can be achieved by the mechanisms of loose scanning and reinitiation (Fig. 3-15).101 Loose scanning is believed to occur when the most 5′ AUG codon is not in a strongly favorable context; it allows the 40S ribosomal subunit to continue scanning until it encounters a more favorable AUG downstream. In the loose scanning mechanism, both translational start codons are used. In contrast, the mechanism of translational reinitiation involves the termination of translation followed by reinitiation of translation at a downstream start codon. Two proteins are encoded from the same mRNA by a start-and-stop mechanism. The process of translational reinitiation can occur by continued scanning of the 40S ribosomal subunit after termination of translation followed by reinitiation, as in loose scanning, or by complete dissociation of the ribosomal subunits at the time of termination followed by complete reassembly at a downstream start codon, referred to as an internal ribosomal entry site (IRES). This use of alternative translation start codons occurs in mRNAs encoding certain classes of transcription factors, as illustrated by the basic leucine zipper (bZIP) proteins CREB, CREM, and the alpha (CEBPA) and beta (CEBPB) isoforms of CCAAT/ enhancer binding proteins (CEBPs). In all four of these DNA-binding proteins, the alternative use of internal start codons results in a switch from activators to repressors. The CREB gene uses translational reinitiation by the somewhat novel mechanism of alternative exon switching that occurs during spermatogenesis.102 At developmental stages IV and V of the seminiferous tubule of the rat, an exon called exon W is spliced into the CREB mRNA. Exon W introduces an in-frame stop codon, thereby terminating translation approximately 40 amino acids upstream of the DNA-binding domain.103,104 Translation is then reinitiated at each of two downstream start codons, resulting in the synthesis of two repressor or inhibitor isoforms of CREB, called I-CREBs. The I-CREBs are powerful dominant negative inhibitors of activator forms of CREB and CREM

because they consist of the DNA-binding domain devoid of any trans-activation domains.102-104 The function of the N-terminal truncated protein consisting of the activation domains devoid of the DNA-binding domain is unknown. It has been postulated that the role of alternative splicing of exon W in the CREB pre-mRNA is to interrupt a forward positive-feedback loop during spermatogenesis. The CREM, CEBPA, and CEBPB mRNAs use alternative downstream start codons to synthesize repressors during development. Like the I-CREBs, these repressors consist of the DNA-binding domains and lack trans-activation domains. The CREM repressor (S-CREM) is expressed during brain development.94 The CEBPA-30 and CEBPA -20 isoforms are expressed during differentiation of adipoblasts to adipocytes, and the CEBP repressor, liver inhibitory protein, is expressed during the development of the liver.94

Post-translational Processing A fifth level of gene expression at which diversification of biologic information can take place is posttranslational processing. Many precursors of polypeptide hormones, particularly those encoding small peptides, contain multiple peptides that are cleaved during posttranslational processing of the prohormones.105 Certain polyprotein precursors, however, contain several copies of the peptide. Examples of prohormones that contain multiple identical peptides are the precursors encoding TRH106 and the α-factor of yeast,107 each of which contains four copies of their respective peptide. Polyproteins that contain several distinct peptides include proenkephalins,108 POMC,109 and proglucagon.110 In many instances, biologic diversification at the level of post-translational processing occurs in a tissue-specific manner. The processing of POMC differs markedly in the anterior and intermediate lobes of the pituitary gland. In the anterior pituitary, the primary peptide products are ACTH and β-endorphin, whereas in the intermediate lobe of the pituitary, one of the primary products is α-melanocytestimulating hormone. The smaller peptides produced are extensively modified by acetylation and phosphorylation of amino acid residues. The processing of proglucagon in the pancreatic alpha cells and in the intestinal L cells is different (see Fig. 3-15).42 In the pancreatic alpha cells, the predominant bioactive product of the processing of proglucagon is glucagon itself; the two glucagon-like peptides are not processed efficiently from proglucagon in the alpha cells and are biologically inactive by virtue of having N-terminal and C-terminal extensions. In the intestinal L cell, the glucagon immunoreactive product is glicentin, a molecule that consists of the N-terminal extension of the proglucagon plus glucagon and the small C-terminal peptide known as intervening peptide 1. Because glicentin has no glucagon-like biologic activity, the bioactive peptide (or peptides) in the intestinal L cells must be one or both of the glucagon-like peptides. In fact, GLP1(7-37), the 31-amino-acid shortened form of glucagon-like peptide 1, is a potent insulinotropic hormone in its actions of stimulating insulin release from pancreatic beta cells.111 This peptide is released from the intestines into the bloodstream in response to oral nutrients and appears to be a potent intestinal incretin factor implicated in the augmented release of insulin in response to oral compared with systemic (intravenous) nutrients. The potential for diversification of biologic information provided by the alternative pathways of gene expression is especially impressive because these pathways can occur in multiple combinations.

Genetic Control of Peptide Hormone Formation   47

Small Regulatory RNAs: MicroRNAs and Small Interfering RNAs The estimated 25,000 protein-coding genes contained in the human genome comprise only 1.25% of transcribed sequences. More than 98% of the transcriptional output of the genome consists of non–protein-coding, potentially regulatory RNAs.112 The transcription of non–proteincoding RNAs from intronic and intergenic regions of the genome has been recognized for several decades, but it was thought to represent “transcriptional noise,” the production of transcripts without function from so-called selfish DNA (i.e., DNA that replicates itself but has no function). In the past few years, however, important functions in development and disease have been identified for the class of small (21 to 25 nucleotides) transcribed RNAs known as microRNAs (miRNAs) and small interfering RNAs (siRNAs). These small regulatory RNAs negatively regulate gene expression at the post-transcriptional level by antisense hybridization to mRNAs. This process either destabilizes mRNAs or inhibits the translation of mRNAs into proteins. About 1000 miRNAs and several thousand siRNAs have been identified in mammalian cells. Between 1% and 3% of the genome encodes small regulatory RNAs, and they are thought to be involved in the regulation of up to 30% of the protein-coding genes.113 One miRNA or siRNA can repress the expression of as many as 100 mRNAs. Although it was initially thought that small regulatory RNAs were ubiquitously expressed in tissues, it is now appreciated that their expression is highly regulated and is cell and tissue specific. These small regulatory RNAs play crucial roles in the regulation of gene expression during embryogenesis and organogenesis and in the control of cell proliferation, apoptosis, differentiation, lineage commitment, and metabolism. The genes encoding miRNAs and siRNAs are transcribed as hairpin-looped RNA precursors (referred to as primary miRNAs or pri-miRNAs); these are initially cleaved in the nucleus by Drosha, a ribonuclease that removes a stem loop, thereby producing an intermediate pre-miRNA hairpin of about 70 nucleotides. These duplex pre-miRNA intermediates are exported from the nucleus by the proteins Exportin 5 and RAN-GTP to the cytoplasm, where they are cleaved by the endonuclease Dicer to form the active miRNAs of about 22 nucleotides that become part of the RNA-induced silencing complex (RISC). The duplex miRNAs are bound by the protein argonaute 2 (EIF2C2). One of the strands is eliminated, and the other (antisense) strand is retained as the mature miRNA. MiRNAs function by imperfect base pairing to the 3′ untranslated regions of their target mRNAs, resulting in mRNA degradation or inhibition of translation. MiRNAs are expressed in spatiotemporal patterns to regulate biologic processes at distinct stages of development. As a consequence of their highly specific expression patterns, miRNAs have been implicated in several diseases, including cancer, neurologic disorders, asthma, cardiovascular disorders, viral infections, and diabetes. MiRNAs may be involved in epigenetic programming of the genome. Convincing evidence indicates that miRNAs in plants and worms have a role in transcriptional gene silencing and do so by contributing to the regulation of histone methylation. A role for miRNAs in gene silencing in mammalian cells is controversial. However, studies suggest that argonaute 2 is required for transcriptional silencing in human cells.114 The importance of miRNAs in diseases of endocrine systems has been investigated. Diabetes is a complex endocrine disease involving myriad factors that modulate the

growth, survival, and regeneration of the insulin-producing beta cells of the pancreas and the sensitivities of peripheral tissues to the actions of insulin. One revealing study demonstrated the importance of miRNAs in the development of the endocrine cells of the pancreas through knockout of Dicer.115

Low Numbers of Expressed Genes in Murine and Human Genomes Sequencing of the human and mouse genomes revealed that each contain approximately 30,000 genes. This number was viewed as remarkably low because the number of genes in the yeast (Saccharomyces cerevisiae), worm (Caenorhabditis elegans), and fly (Drosophila melanogaster) genomes is about 20,000. However, tissue-specific alternative exon splicing and alternative promoter use occur much more frequently in humans and mice than in yeast, worms, and flies. The complexities of the mRNAs expressed in humans and mice are exemplified by the growing database of expressed sequence tags, and it seems reasonable to extrapolate that the human genome may express as many as 100,000 to 200,000 mRNAs that encode proteins with distinct, specific functions. This interpretation is based on the observation that alternative exon splicing and promoter are 5 to 10 times more frequent in higher vertebrate mammals than in yeasts and flies.

Genome-Wide Association Studies for Identification of Complex-Trait Disease Genes Determination of the nucleic sequence of the human genome and mapping of the locations of expressed genes have created opportunities for the identification of genes responsible for complex-trait diseases,116,117 such as diabetes, obesity, hypertension, asthma, schizophrenia, and bipolar disease. The genetic background of complex-trait diseases consists of multiple genes that collectively account for the disease phenotype. Genome-wide association studies (GWAS) enable identification of marker loci that co-segregate with the disease trait loci and association of the marker with risk for the disease. GWAS do not necessarily reveal the cause of disease, but they can identify gene loci that modify risk for the development of disease. Association studies are based on the assumption that recombination (rearrangements) of DNA sequences in the genome are relatively slow to occur in the population, so that experimentally detectable marker loci consisting of single-nucleotide polymorphisms (SNPs) remain in place with respect to the disease loci. A large population (1000 to 10,000 individuals) with a high prevalence of the disease trait phenotype can be segregated by the presence or absence of the disease trait into cases and controls, respectively. The DNA from cases is screened for the presence of SNPs by high-output DNA array analyses using common SNPs present in the population genome. There are an estimated 10 million random polymorphisms in the genomes of the general human population (approximately 1 SNP per 100 base pairs of DNA). Currently available high-density SNP libraries contain up to 770,000 SNPs spanning 74% of the genome, making it possible to identify specific SNPs that lie near the disease locus being studied.118 Examination of maps of known genes in the region of the associated SNP or SNPs can help to identify potential candidate genes that may be associated with the disease. The polymorphism may represent mutation in the protein-coding region (exons) of the disease gene

48   Genetic Control of Peptide Hormone Formation itself, but more often, the SNPs reside in regions of DNA within the gene (i.e., introns) or flanking the gene. GWAS have identified many genes potentially associated with disease and have provided insights into pathogenesis and possible drug targets. Because of the limitations of the method—the need for very large study populations and the fact that only alleles associated with common SNPs in the population are identified—only a modest fraction of the genetic contribution to common diseases has been delineated.119-121 Type 2 diabetes provides a useful example of what GWAS have accomplished. Screening of large populations in which the prevalence of type 2 diabetes is high (approaching 10%) has uncovered 19 candidate genes to date.122 One of these genes, the transcription factor TCF7L2, is strongly associated with the existence and predictive development of insulin-deficient type 2 diabetes in multiple populations. Carriers of the TCF7L2 risk alleles have impaired glucose-stimulated insulin secretion and diminished augmentation of insulin secretion by incretin hormones such as GLP1 and gastric inhibitory polypeptide.123 TCF7L2 is the predominant DNA-binding protein component of beta-catenin/TCF regulators of gene transcription in beta cells, and beta-catenin/TCF7L2 constitutes the downstream effector of the WNT signaling pathway stimulated by the incretin hormone GLP1 and the chemokine CXCL12 (formerly called SDF1).124 WNT signaling mediated by GLP1 and CXCL12 is required for beta cell growth and survival, respectively.

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Ligands That Act through Nuclear Receptors,  52 Nuclear Receptor Signaling Mechanisms,  54 Receptor Regulation of Gene Transcription,  57

CHAPTER CHAPTER 4  Mechanism of Action of Hormones That Act on Nuclear Receptors MITCHELL A. LAZAR

Hormones can be divided into two groups on the basis of where they function in a target cell. The first group includes hormones that do not enter cells; instead, they signal by means of second messengers generated by interactions with receptors at the cell surface. All polypeptide hormones (e.g., growth hormone), monoamines (e.g., serotonin), and prostaglandins (e.g., prostaglandin E2), use cell surface receptors (see Chapter 5). The second group, the focus of this chapter, includes hormones that can enter cells. These hormones bind to intracellular receptors that function in the nucleus of the target cell to regulate gene expression. Classic hormones that use intracellular receptors include thyroid and steroid hormones. Hormones serve as a major form of communication between different organs and tissues. They allow special­ ized cells in complex organisms to respond in a coor­ dinated manner to changes in the internal and external environments. Classic endocrine hormones are secreted by endocrine glands and are distributed throughout the body through the bloodstream. These hormones were discovered by purifying the biologically active substances from clearly definable glands.

Numerous other signaling molecules share with thyroid and steroid hormones the ability to function in the nucleus to convey intercellular and environmental signals. Not all of these molecules are produced in glandular tissues. Although some signaling molecules, such as classic endo­ crine hormones, arrive at target tissues through the blood­ stream, others have paracrine functions (i.e., acting on adjacent cells) or autocrine functions (i.e., acting on the cell of origin). In addition to the classic steroid and thyroid hormones, lipophilic signaling molecules that use nuclear receptors include derivatives of vitamins A and D, endogenous metabolites such as oxysterols and bile acids, and nonnatural chemicals encountered in the environment (i.e., xenobiotics). These molecules are referred to as nuclear receptor ligands. The nuclear receptors for all of these signal­ ing molecules are structurally related and are collectively referred to as the nuclear receptor superfamily.1-3 The study of these receptors is a rapidly evolving field, and more detailed information can be obtained by visiting the Nuclear Receptor Signaling Atlas web site (http:// www.nursa.org [accessed September 2010]). 51

52   Mechanism of Action of Hormones That Act on Nuclear Receptors

LIGANDS THAT ACT THROUGH NUCLEAR RECEPTORS General Features of Nuclear Receptor Ligands Unlike polypeptide hormones that function through cell surface receptors, no ligands for nuclear receptors are directly encoded in the genome. All nuclear receptor ligands are small (molecular mass 1000) that bind to GPCRs, it is not surprising that considerable diversity is evident in the sequence and structure of presumptive GPCR ligand– binding domains. The opsins are unique among GPCRs in that the ligand, retinal, is covalently bound to a lysine in the seventh transmembrane helix.127 Ligand binding for other members of family 1 with a short extracellular N-terminus, such as adrenergic and other monoamine receptors, involves a pocket within the transmembrane helices, as has been demonstrated for rhodopsin (see Fig. 5-10). Rhodopsin’s N-terminus and extracellular loops form a well-structured domain that occludes the retinal binding site. The hormone receptors have a more open extracellular structure that facilitates ligand entry into the transmembrane domain.127 For other family 1 GPCRs, the extracellular N-terminus, perhaps together with extracellular loops and portions of the transmembrane helices, is involved in ligand binding. In the case of the glycoprotein hormone receptors, the large, curved, extracellular N-terminus plays the principal role in hormone binding.129 In a model for peptide binding to family 2 receptors, the extracellular N-terminus is responsible for initial binding to the peptide C-terminus, followed by peptide N-terminus binding to the seven-transmembrane domain.130 For family 3 GPCRs, the three-dimensional structure of the metabotropic glutamate receptor shows that agonist binding occurs within a cleft between the lobes of the Venus flytrap.128

G Protein Coupling Because the number of potential G proteins to which GPCRs couple is much more limited than the number of ligands that bind GPCRs, more conservation of the domains involved in G protein coupling is expected. Although GPCRs can be broadly divided into those that couple to Gs, those that couple to the Gq subfamily, and those that couple to the Gi-Go subfamily, the situation is probably more complicated.124 Specificity of coupling to the most recently identified G proteins, Gl2 and G13, is still uncertain. Also, some GPCRs evidently can couple to both Gs and Gq. Many studies have been performed to define the sites of ligand binding and G protein coupling of GPCRs.124,131 Considerable evidence points to the third intracellular loop (particularly its membrane-proximal portions) and to the membrane-proximal portion of the C-terminus as key determinants of G protein–coupling specificity. For example, exchanging only the third intracellular loop

between different GPCRs confers the G protein–coupling specificity of the exchanged loop on the recipient GPCR.132 In contrast, the second intracellular loop, although important for G protein coupling, appears to play a role in the activation mechanism rather than in determining specificity of coupling.132 A tripeptide “DRY” motif (D/E, R, Y/W) at the juncture between the third transmembrane helix and the start of the second intracellular loop that is highly conserved in family 1 GPCRs is critical for G protein activation.131 The three-dimensional structure of a complex between activated rhodopsin and a C-terminal peptide of the retinal G protein α–subunit shows direct binding of the latter to the arginine of the receptor’s DRY motif.127

Mechanism of Activation The precise mechanism of activation after agonist binding has not been defined for most GPCRs, but x-ray crystallographic studies of rhodopsin, β-adrenergic, and adenosine receptors provide the clearest picture available. In the ground state, retinal covalently bound to the seventh transmembrane helix in rhodopsin holds the transmembrane helices in an inactive conformation, at least in part through so-called ion lock created by electrostatic interaction between the arginine at the end of the third transmembrane domain and a glutamate at the end of the sixth transmembrane domain.127 Isomerization of retinal on absorption of light of the appropriate wavelength converts an antagonist ligand into an agonist. The rhodopsin crystal structure identifies the residues in the transmembrane helices that interact with retinal and suggests a mechanism for movement of the helices, particularly the third and sixth transmembrane domains, on photoactivation of retinal.127 Movement of the transmembrane helices leads to changes in conformation of cytoplasmic loops that promote G protein activation. For family 1 receptors related to rhodopsin, determination of their three-dimensional structures validates the idea that a change in conformation of transmembrane helices is the direct result of agonist or antagonist binding to residues within the helices.127 Molecular modeling by computer on the basis of the rhodopsin structure and experimental testing offers a useful approach to defining critical structural regions of other GPCRs.133 The availability of three-dimensional structures for β-adrenergic and adenosine receptors also permits in silico studies of ligand binding to these receptors.127 For other GPCRs whose presumptive site of agonist binding does not solely involve direct contact with transmembrane helices (i.e., families 2 and 3 and the glycoprotein hormone receptors in family 1), much remains to be learned about how agonist binding to the extracellular domain of these GPCRs leads to presumptive changes in conformation of transmembrane helices and receptor activation. Determination of the structure of the extracellular domain of the FSH receptor bound to FSH illustrates the general mechanism of glycoprotein hormone binding to cognate GPCRs and resultant interactions with the seventransmembrane domain receptor leading to activation. A hypothesis of GPCR activation postulates that GPCRs are in equilibrium between an activated state and inactive state. These states presumably differ in the disposition of the transmembrane helices and the cytoplasmic domains that determine G protein coupling. According to this ternary complex model, agonists are viewed as stabilizing the activated state. Antagonists may be neutral; that is, they may simply compete with agonists for receptor binding, with their binding having no influence on the equilibrium. Alternatively, they may be inverse agonists,

Mechanism of Action of Hormones That Act at the Cell Surface   77

with their binding stabilizing the inactive state of the receptor.134 Multiple conformational states may exist between the so-called active and inactive conformations, with different ligands capable of promoting a particular conformation. This possibility has important implications for the downstream signaling consequences of a given ligand, a concept called ligand-directed signaling124 or ligandbiased efficacy.

Dimerization Members of the tyrosine kinase receptor family require dimerization as part of their activation mechanism, and many GPCRs form homodimers and heterodimers.122,124 Residues within transmembrane helix 6 may foster dimerization of small, family 1 GPCRs,136 and intermolecular disulfide bonds in the extracellular N-terminal domain are involved in homodimerization of most family 3 GPCRs.128,137,138 A coiled-coil interaction in the C-terminus of γ-aminobutyric acid B receptor subtypes is responsible for heterodimerization, and this is critical for proper receptor function.139 Modifications of ligand binding, signaling, and receptor sequestration have been demonstrated on heterodimerization of angiotensin with bradykinin receptors, of κ with δ opioid receptors, and of opioid with β-adrenergic receptors, and a role for ligand-independent action of heterodimerized orphan GPCRs (discussed later) has been proposed.140 Additional studies are needed to elucidate the physiologic relevance of GPCR homodimerization and heterodimerization.

G Protein–Coupled Receptor Desensitization Pharmacologists have long appreciated that continued exposure to agonist leads to a diminished response, or desensitization. This phenomenon has been extensively studied in GPCRs. Two forms are defined: heterologous desensitization, in which binding of agonist to one GPCR leads to a diminished response of a different GPCR to its agonist, and homologous desensitization, in which desensitization occurs only for the GPCR to which agonist is bound. Both forms involve GPCR phosphorylation but by different kinases and at different sites. Stimulation of cyclic adenosine monophosphate formation by agonist binding to a Gs-coupled GPCR leads to activation of protein kinase A, which in turn can phosphorylate and desensitize the GPCR. Phosphorylation may also alter G protein–coupling specificity.122 Similarly, protein kinase C activation resulting from GPCR coupling to Gq family members may cause protein kinase C–catalyzed phosphorylation of GPCRs with desensitization. In retinal photoreceptors, a specific rhodopsin kinase and the arrestin protein have been implicated in attenuation of the light response. Just as there are parallels between rhodopsin and GPCR structure, parallels have also been identified in desensitization mechanisms. Rhodopsin kinase is only one member of a family of GPCR kinases and arrestin only one of a family of related proteins that function in desensitization of many members of the GPCR superfamily.126 GPCR kinases preferentially phosphorylate the agonist-bound form of a GPCR, ensuring homologous desensitization. On GPCR phosphorylation by GPCR kinase, arrestins bind to the third intracellular loop and C-terminal tail of the GPCR, thereby blocking G protein binding (see Fig. 5-11). GPCR kinases and arrestins not only desensitize GPCRs but also mediate other functions, including receptor internalization and interaction with other effectors.

G PROTEIN–COUPLED RECEPTOR INTERACTIONS WITH OTHER PROTEINS The initial paradigm of GPCR function postulated that G protein activation is the sole outcome of agonist binding to GPCRs. With the identification of GPCR interactions with GPCR kinases and arrestins, this concept was modified to include the proteins involved in GPCR desensitization. Later evidence suggested that GPCR interaction with arrestins may also permit recruitment of other proteins to the GPCR. For example, the SRC tyrosine kinase may interact with the β-adrenergic receptor, with β-arrestin serving as an adaptor.141 Arrestins may also recruit proteins involved in endocytosis, and GPCR kinases may recruit additional signaling proteins to the GPCR.141 Other classes of proteins may interact with specific GPCRs without recruitment by GPCR kinases and arrestins. They include SH2 domain–containing proteins, small GTPbinding proteins, and postsynaptic density protein-95/ discs large/zona occludens 1 (PDZ) domain–containing proteins. An examples of the latter is binding of the Na+/ H+ exchanger regulatory factor to the C-terminus of the β-adrenergic receptor.141 The long C-terminus of family 3 GPCRs (e.g., metabotropic glutamate receptors) contains polyproline motifs involved in binding members of the Homer family of PDZ proteins, which can facilitate functional interactions with other proteins such as the inositol triphosphate receptor.142 Receptor activity-modifying proteins (RAMPs), a family of single-transmembrane-domain proteins, appear to heterodimerize with certain GPCRs, assisting them in proper folding and membrane trafficking.143 This rapidly evolving aspect of GPCR function promises many interesting developments.

G PROTEIN–COUPLED RECEPTORS IN DISEASE PATHOGENESIS AND TREATMENT Because of their diverse and critical roles in normal physiology, their accessibility on the cell surface, and their ability to synthesize selective agonists and antagonists, GPCRs have been a major target for drug development. One estimate is that about 65% of prescription drugs target GPCRs. Drugs targeting GPCRs may act as agonists, antagonists, or allosteric modulators. For example, calcimimetic drugs inhibit PTH release by binding to the seventransmembrane domain and acting as positive allosteric modulators of the Ca2+-sensing receptor.138 With the cloning of GPCR cDNAs, much greater diversity of receptor subclasses became evident than had been anticipated on the basis of pharmacologic studies. For example, five muscarinic receptor subtypes and an even greater number of serotoninergic GPCRs were identified.130 This information has enabled development of highly specific subtypeselective drugs that have fewer side effects than those produced by previously available agents. Another result of the cloning of GPCR cDNAs by homology screening and polymerase chain reaction– based approaches is the identification of orphan GPCRs, receptors that have the canonical, predicted seventransmembrane-domain structure of GPCRs but have no identified physiologic agonists. There have been substantial efforts to identify the relevant ligands for these orphan

78   Mechanism of Action of Hormones That Act at the Cell Surface receptors.140 For example, the Krebs cycle intermediates, succinate and α-ketoglutarate, were shown to be the physiologically relevant activators of the orphan GPCRs, GPR91, and GPR99, respectively; through binding, they regulate renin release and blood pressure.144 In addition to revealing novel physiologic and pathophysiologic mechanisms, GPCR “deorphanization” provides novel targets for drug development. Beyond drug development, defects in GPCRs are an important cause of a variety of human diseases.145 GPCR mutations can cause loss of function by impairing any of several steps in the normal GPCR-GTPase cycle (see Fig. 5-11), including failure to synthesize the GPCR protein altogether, failure of synthesized GPCR to reach the plasma membrane, failure of GPCR to bind or be activated by agonist, and failure of GPCR to couple to or activate G protein. Because in most cases clinically significant impairment of signal transduction requires loss of both alleles of a GPCR gene, most of these diseases are inherited in autosomal recessive fashion (Table 5-1). Most diseases manifest as resistance to the action of the normal agonist and mimic deficiency of the agonist. For example, TSH receptor loss-of-function mutations cause a form of hypothyroidism that mimics TSH deficiency, but the serum TSH level is elevated in such cases, reflecting resistance to the hormone’s action caused by defective receptor function. Hypogonadotropic hypogonadism may be caused by loss-of-function mutations of the GnRH receptor, the orphan GPR54, or the prokineticin receptor 2 (PROKR2). In the first case, there is resistance to the action of GnRH. In the latter two cases, hypogonadotropic hypogonadism may reflect failure to release GnRH, but the precise mechanism has not been defined.146,147 Nephrogenic diabetes insipidus (i.e., renal vasopressin resistance) is caused by loss-of-function mutations in the arginine vasopressin receptor 2 gene (AVPR2) located on the X chromosome. Males with a single copy of the gene develop the disease when they inherit a mutant gene; most females do TABLE 5-1 

Diseases Caused by G Protein–Coupled Receptor Loss-of-Function Mutations Receptor

Disease

Inheritance

V2 vasopressin ACTH GHRH GnRH GPR54 Prokineticin receptor 2 FSH

Nephrogenic diabetes insipidus Familial ACTH resistance Familial GH deficiency Hypogonadotropic hypogonadism Hypogonadotropic hypogonadism Hypogonadotropic hypogonadism

X-linked AR AR AR AR AD*

Hypergonadotropic ovarian   dysgenesis Male pseudohermaphroditism Familial hypothyroidism Familial hypocalciuric hypercalcemia Neonatal severe primary hyperparathyroidism Obesity Blomstrand chondrodysplasia

AR

LH TSH Ca2+ sensing Melanocortin 4 PTH/PTHrP

AR AR AD AR AR AR

*With incomplete penetrance. ACTH, adrenocorticotropic hormone; AD, autosomal dominant; AR, autosomal recessive; FSH, follicle-stimulating hormone; GH, growth hormone; GHRH, growth hormone–releasing hormone; GnRH, gonadotropin-releasing hormone; GPR54, orphan G protein–coupled receptor 54; LH, luteinizing hormone; PTH, parathyroid hormone; PTHrP, parathyroid hormone–related protein; TSH, thyroid-stimulating hormone.

TABLE 5-2 

Diseases Caused by G Protein–Coupled Receptor Gain-of-Function Mutations Receptor

Disease

Inheritance

LH TSH

Familial male precocious puberty Sporadic hyperfunctional thyroid nodules Familial nonautoimmune hyperthyroidism Familial hypocalcemic hypercalciuria Jansen’s metaphyseal   chondrodysplasia Nephrogenic inappropriate   antidiuresis

AD Noninherited (somatic) AD

Ca2+ sensing PTH/PTHrP V2 vasopressin

AD AD AD

AD, autosomal dominant; LH, luteinizing hormone; PTH, parathyroid hormone; PTHrP, parathyroid hormone–related protein; TSH, thyroid-stimulating hormone.

not show overt disease because random X inactivation leaves them with an average of 50% of normal gene function. Most AVPR2 mutations associated with nephrogenic diabetes insipidus cause loss of function by impairing normal synthesis or folding of the receptor, or both. A novel mechanism for receptor loss of function elucidated for an AVPR2 missense mutation associated with nephrogenic diabetes insipidus involves constitutive arrestinmediated desensitization.148 The extracellular Ca2+-sensing receptor appears to be an interesting exception to the association between GPCR loss-of-function mutations and hormone resistance. Lossof-function mutations of this receptor mimic a hormone hypersecretion state, primary hyperparathyroidism. Such mutations do cause hormone resistance, but extracellular Ca2+ is the hormonal agonist that acts through this receptor to inhibit PTH secretion. A loss-of-function mutation of one copy of the receptor gene typically causes mild resistance to extracellular Ca2+, which manifests as familial hypocalciuric hypercalcemia. If two defective copies are inherited, extreme Ca2+ resistance causing neonatal severe primary hyperparathyroidism results (see Table 5-1). In some cases, a heterozygous receptor loss-of-function mutation is associated with neonatal severe primary hyperparathyroidism, perhaps reflecting a dominant negative effect caused by dimerization of wild-type and mutant receptors.149 GPCR gain-of-function mutations are an important cause of disease (Table 5-2).145 Given the dominant nature of activating mutations, most of these diseases are inherited in an autosomal dominant manner. Activating TSH receptor mutations may be inherited in autosomal dominant fashion and cause diffuse thyroid enlargement in familial nonautoimmune hyperthyroidism, or they may occur as somatic mutations causing focal, sporadic, hyperfunctional thyroid nodules.150 Activating germline LH receptor mutations cause familial male precocious puberty due to LH-independent Leydig cell hyperfunction, and somatic LH receptor mutations may cause focal Leydig cell tumors.150 Unlike loss-of-function mutations, which may be missense, nonsense, or frameshift mutations that truncate the normal receptor protein, GPCR gain-of-function mutations are almost always missense mutations. The location and nature of naturally occurring, disease-causing mutations offer important insights into GPCR structure and function. The basis for defective receptor function is clear with mutations that truncate receptor synthesis prematurely. More

Mechanism of Action of Hormones That Act at the Cell Surface   79

subtle missense mutations may impair function if they involve highly conserved residues in transmembrane helices critical for normal protein folding. Activating missense mutations often involve residues within or bordering transmembrane helices and are thought to disrupt normal inhibitory constraints that maintain the receptor in its inactive conformation.127,151 Mutations disrupting these constraints mimic the effects of agonist binding and shift the equilibrium toward the activated state of the receptor. A striking example is the activating missense mutations in AVPR2 of arginine 137, part of the DRY motif at the intracellular border of transmembrane helix 3 that is conserved in most family 1 GPCRs, which lead to the syndrome of nephrogenic inappropriate antidiuresis.152 Diseases caused by activating GPCR mutations clinically mimic states of agonist excess, but direct measurement shows that agonist concentrations are low, reflecting normal negative feedback mechanisms. The Ca2+-sensing receptor is an apparent exception, with activating mutations causing functional hypoparathyroidism. Although in most GPCRs, disease-associated gain-of-function mutations cause constitutive, agonist-independent activation, such mutations in the Ca2+-sensing receptor, with rare exceptions, cause increased sensitivity to extracellular Ca2+ rather than to Ca2+-independent activation.149 Naturally occurring animal models of human disease have revealed additional examples of etiologic GPCR mutations. For example, a loss-of-function mutation in the hypocretin (orexin) type 2 receptor gene was identified in canine narcolepsy.153 Dozens of mouse GPCR gene knockout models have been created, and many have revealed interesting and unexpected phenotypes. Characterization of the phenotype resulting from disruption of a mouse GPCR gene may accurately predict the clinical picture resulting from the corresponding mutation in humans; examples include disruption of the melanocortin 4 receptor gene, which results in obesity154 and disruption of the PTH/PTH-related protein receptor gene, which impairs normal bone growth and development in mice155 and in the human disease known as Blomstrand chondrodysplasia.156 Knockouts of orphan GPCR genes can help reveal their physiologic functions. For example, knockout of the mouse gene encoding an orphan GPCR (GPR48) demonstrated its role in regulation of bone formation and remodeling.157 Studies of a constitutively active GPR3 in the mouse and in Xenopus revealed a role in maintaining meiotic arrest in oocytes.158 Availability of mouse knockout models of human diseases should facilitate testing of novel therapies, including gene transfer. For example, aminoglycosides, which suppress premature termination codons, were shown to rescue expression and function in mice with nephrogenic diabetes insipidus caused by a nonsense mutant in AVPR2.159 Many disease-causing loss-of-function mutations in GPCRs lead to defective protein folding or protein routing. Novel therapeutic approaches such as use of molecular chaperones and modulation of a cell’s quality-control mechanisms have shown promise in in vitro studies.160 Screening of GPCR genes for mutations as the potential cause of human disorders may continue to identify more examples, but it is also becoming clear that variations in GPCR gene sequence can have profound consequences beyond causing resistance to or activation independent of the cognate hormone agonist. For example, familial spontaneous ovarian hyperstimulation syndrome occurring in early pregnancy was shown to be caused by missense mutations in the transmembrane helix domain of the FSH receptor.161 These mutations increase receptor basal activity and

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Types of Assays,  84 Analytic Validation,  92 Quality Assurance,  95

CHAPTER CHAPTER 6  Laboratory Techniques for Recognition of Endocrine Disorders GEORGE G. KLEE

Endocrinology is a practice of medicine that is highly dependent on accurate laboratory measurements because small changes in hormone levels often may be more specific and more sensitive for early disease than the classic physical signs and symptoms. Most endocrinologists currently do not have facilities to develop and validate laboratory assays; therefore they rely on commercial analytic assays or send a patient’s specimen to specialized laboratories. Even most hospital and commercial laboratories have minimal expertise for developing analytic assays. This critical dependence on quality laboratory measurements, combined with minimal information about the performance of these tests, places endocrinologists in a potentially vulnerable position. This chapter provides an overview of the strengths and weaknesses of the analytic techniques typically used for endocrine measurements in blood and urine. Concentrations of most hormones are much lower than those of general chemistry analytes, and specialized techniques are necessary to measure these low concentrations.

Two major types of assays for measuring hormones are described: immunoassays (both competitive and sandwich) and chromatographic assays with various detection systems including mass spectrometry. Also, a brief overview is provided for selected nucleic acid–based assays for evaluation of genetic alterations. The analytic performance validation required by the U.S. government for laboratories testing specimens of Medicare patients is outlined, along with explanations of these performance parameters. This information should help endocrinologists better assess the performance of the analytic systems that they are using. Techniques to investigate discordant laboratory test values also are presented to help clinicians work with their specific laboratories to reconcile test values that do not match clinical presentations. Hormone concentrations are reported in molar units, mass units, or standardized units, such as World Health Organization (WHO) International Units (IU). When these measurements are expressed in molar units, most 83

84   Laboratory Techniques for Recognition of Endocrine Disorders T4 Cortisol Vitamin D-25-OH Progesterone DHEA T3 Testosterone—male Norepinephrine FSH Prolactin Testosterone—female Vitamin D 1,25-OH Estradiol—female Estradiol—male LH Aldosterone TSH FT4 Insulin FT3 Epinephrine Growth hormone 1

100

10,000

1,000,000

Concentration in picomoles/L Figure 6-1  Six-logarithm range of normal plasma concentrations in endocrine tests. DHEA, dehydroepiandrosterone; FSH, follicle-stimulating hormone; FT4, free thyroxine; FT3, free triiodothyronine; LH, luteinizing hormone; T3, triiodothyronine; T4, thyroxine; TSH, thyrotropin.

hormones in blood and urine are present in concentrations of 10−6 to 10−12 mol/L (Fig. 6-1). The terms used to describe these concentrations are micromolar (10−6 mol/L), nanomolar (10−9 mol/L), and picomolar (10−12 mol/L). The range—from the lowest to the highest concentrations—is more than a million-fold difference. Therefore, laboratory techniques must be targeted to the levels of each given hormone. The major techniques for measuring the lower picomolar concentrations are immunoassay and mass spectrometry, whereas the higher nanomolar and micromolar concentrations can be measured by these methods or by optical density chromatography and chemical detection systems. Some hormones, such as thyrotropin (thyroid stimulating hormone, or TSH), have very low concentrations, in the femtomolar (10−15 mol/L) range, in patients with diseases such as thyrotoxicosis. Exquisitely sensitive immunometric assays are usually used to measure these very low concentrations.1,2

TYPES OF ASSAYS The major techniques used for steroid and protein measurements are (1) antibody-based immunologic assays, of which there are two subcategories—competitive immunoassays and immunometric (sandwich) assays, and (2) chromatographic assays with various detectors including mass spectrometers.

Competitive Immunoassays The term competitive radioimmunoassay refers to a measurement method in which an antigen (e.g., a hormone) in a specimen competes with radiolabeled reagent antigen for a limited number of binding sites on a reagent antibody. The three basic components of a competitive immunoassay are (1) antiserum specific for a unique epitope on a hormone or antigen, (2) labeled antigen that binds to this antiserum, (3) unlabeled antigen in the specimen or standard that is to be measured.3,4 The antiserum is diluted to a concentration at which the number of binding sites available on the antibodies is fewer than the number of antigen molecules (labeled and unlabeled) in the reaction mixture. The labeled and unlabeled antigens compete for this limited number of binding sites on the antiserum. The competition is not always equal because the labeled antigen (tracer) and the native antigen may react differently with the antibody. This disparity in reactivity may be caused by alteration of the antigen due to the chemical attachment of the label or by differences in the endogenous antigen compared with the form of the antigen used in the reagents. As long as the reactions are reproducible, these differences in reactivity are not important because the reaction can be calibrated with standard reference materials having known concentrations. Figure 6-2 illustrates the concepts of a competitive immunoassay. In the schematic diagram, 8 units of antibody react with 16 units of labeled antigen and 4 units of

Laboratory Techniques for Recognition of Endocrine Disorders   85 Competive binding 8 Ab

+

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Figure 6-2  A, Principles of competitive binding assays. Ab, antibody; Ag*, labeled antigen; AG0, native antigen. See text for details. B, Typical doseresponse curve. The point on the curve labeled (B0) represents the percentage binding of the radiolabeled antigen when zero native antigen is present. The nonspecific binding (NSB) level is the minimal binding level of radiolabeled antigen at high concentrations of native antigen.

native antigen. At equilibrium (assuming equal reactivity), 6 units of label and 2 units of native antigen are bound to the limited supply of antibody. The antigen bound to the antibody is separated from the liquid antigen by any of several methods, and the amount of labeled antigen in the bound portion is quantitated (see Fig. 6-2A). The assay is calibrated by measuring standards with known concentrations and cross-plotting the signal (i.e., counts of the gamma rays emitted from the radioactive label) versus the concentration of the standard to generate a dose-response curve. As the concentration increases, the signal decreases exponentially. Generally, the antiserum used in a competitive assay is diluted to a titer that binds between 40% and 50% of the labeled antigen when no unlabeled antigen is present. Further dilution of the antiserum increases the analytic sensitivity but decreases both the signal and the range of the assay. The precision of competitive immunoassays is related to the rate of change of the signal compared with the rate of change of concentration (i.e., the slope of the dose-response curve).5 In Figure 6-2B, the slope is much lower at higher concentrations, causing the assay precision to be less at higher concentrations. Most competitive immunoassays also have a relatively flat dose-response curve at very low

concentrations, causing poor precision at the low end of the assay. Consequently, the precision profile for most immunoassays is U-shaped, having the best coefficients of variation in the center of the dose-response curve. As shown in Figure 6-2, the higher the concentration of unlabeled antigen, the lower the amount of radiolabeled antigen that binds to the limited amount of antiserum. The signal decreases exponentially, from approximately 50% of total counts at zero concentration to a minimum value at high concentrations. This minimal binding, or nonspecific binding (NSB), is a valuable control parameter. Elevations in NSB usually signify impurities in the label that bind to the sides of the tubes and are not competitively displaced. Most assays add surfactants and proteins to minimize the NSB. Monitoring of changes in the NSB provides an early warning of potential assay problems. Statistical data processing techniques are needed to translate the assay signals into concentrations. As illustrated, these dose-response curves typically are not linear, and numerous curve-fitting algorithms have been developed. Before the introduction of microprocessors, tedious, error-prone, manual calculations were required to mathematically transform the data into linear models. A commonly used model was to cross-plot the logit of the normalized signal versus the logarithm of the concentration and to use linear regression lines to establish the doseresponse curve.5 Today, curve fitting usually is accomplished electronically with the use of programs that automatically test the robustness of fit of multiparameter curves after statistically eliminating discordant data points.6,7 However, users of these systems must understand their limitations and should pay attention to any warnings presented by the programs during processing of the data. In radioimmunoassays, radioactive iodine (125I) is usually used to label the antigen. The immune complexes are separated from the unbound molecules by precipitation with centrifugation after reaction with secondary antisera and precipitating reagents (e.g., polyethylene glycol).8 These radioimmunoassays are labor intensive and may require special handling and licensure to ensure safety of the radioisotopes. Because of the statistical counting errors associated with the relatively low radioactive counts and the poor reproducibility associated with the multiple manual steps, most laboratories perform the measurements in duplicate.9 Even when the averages of duplicate measurements are used, many manual radioimmunoassays have coefficients of variation between 10% and 15%. It is important that key quality control parameters for radioimmunoassays be carefully monitored. In addition to NSB, another key quality control parameter is the percentage binding of the radiolabeled when zero native antigen (B0) is present (i.e., when B represents the signal from tracer bound in the sample). As the label deteriorates, because of aging, the binding often decreases, resulting in a less reliable assay. Another important quality control parameter is the slope of the dose-response curve. This parameter can be tracked by monitoring the concentration corresponding to half-maximum binding (50% of B/B0). If this concentration increases significantly, the slope of the response curve decreases and the assay may not be capable of reliably measuring patient specimens at clinically important concentrations. Many commercial kits and automated immunoassays use nonisotopic signal systems to measure hormone concentrations. These assays often use colorimetric, fluorometric, or chemiluminescent signals rather than radioactivity to quantitate the response. The advantages of these signals

86   Laboratory Techniques for Recognition of Endocrine Disorders are biosafety, longer reagent shelf life, and ease of automation. On the other hand, they are more subject to matrix interferences than radioactive iodine. Radioactivity is not affected by changes in protein concentration, hemolysis, color, or drugs (except for other radioactive compounds), whereas many of the current signal systems can yield spurious results when such interferences are present. In addition, many automated immunoassays are read kinetically before the reactions reach equilibrium. This early reading accentuates the effects of any matrix differences between the reference standards and patient specimens. Later in this chapter, potential troubleshooting steps are outlined to help clinicians evaluate the integrity of test measurements when spurious results are suspected. Solid-phase reactions often are used in current immunoassays to facilitate the separation of the bound antibodyantigen complexes from the free reactants.10 Three frequently used solid-phase materials are microtiter plates, polystyrene beads, and paramagnetic particles.11 Typically, the antibody is attached to the solid phase, and the separation of the immune complexes from the unbound moieties is accomplished by plate washers, bead washers, or magnetic wash stations, eliminating the need for centrifugation. Another novel way of accomplishing this separation is to attach high-affinity linkers to antiserum, which then can be coupled to a complementary linker on the solid phase. An excellent pair of linkers is biotin and streptavidin. These compounds bind with affinity constants of approx­ imately 1015 L/M.12 Biotin is a relatively small molecule that can be easily covalently attached to antiserum and used with streptavidin (a 70-kd tetrameric, nonglycosy­ lated protein) conjugated to microtiter plates, beads, or paramagnetic particles to facilitate separation. This technique allows the antibody-antigen reaction to proceed faster with less stearic hindrance than when the antibody is directly coupled to the solid phase. The antiserum used in these assays is a crucial component. Earlier immunoassays used polyclonal antiserum produced in animals. The process of generating these antisera is a combination of art, science, and luck. In general, a relatively pure form of the antigen is conjugated to a carrier protein (especially if the antigen is less than 10 kd), mixed with adjuvant (e.g., Freud’s complete adjuvant), and injected intradermally into the host animal. After several boosts with conjugated protein plus Freud’s incomplete adjuvant, the host animal recognizes the material as foreign and develops immune responses. The antiserum then is harvested from the animal’s blood. Under optimal conditions, moderate quantities of high-affinity antisera that react only with the specific target antigen are developed. The analytic sensitivity of a competitive immunoassay is approximately inversely related to the affinity of the antiserum, such that an antiserum with an affinity constant of 109 L/M can be used to measure analytes in the nanomolar concentration range. The polyclonal antiserum developed by immunizing animals represents a composite of many immunologic clones, with each clone having a different affinity and different immunologic specificity. Most clones have affinities in the 107 to 108 L/M range, with only rare clones having affinities greater than 1012 L/M. Various techniques are used to develop a specific antiserum, including (1) altering the form of the antigen by blocking cross-reacting epitopes and (2) purifying the antiserum by using affinity chromatography to select antibodies directed toward the epitope of interest. Affinity-column purification also can be used

for immunoextraction of higher-affinity antisera by selectively eluting antiserum from the column by means of a series of buffers with increasing acidity.13 The major disadvantage of a polyclonal antiserum is the limited quantity produced. The large quantities needed by commercial suppliers of immunoassay reagents often require them to use multiple sources of antisera. These changes in antisera can cause significant changes in assay performance. In many instances, laboratories and clinicians are not informed about these changes, a situation that may cause problems in medical decisions. Monoclonal antisera are used in many current immunoassays. These antisera are made by immunizing animals (usually mice) using techniques similar to those used for polyclonal antisera; instead of harvesting the antisera from the blood, the animal is euthanized and the spleen is removed.14 The lymphocytes in the spleen are fused with myeloma cells to make cells that will grow in culture and produce antisera. These fused cells are separated into clones by means of serial plating techniques similar to those used in subculturing bacteria. The supernatant of these monoclonal cell lines (or ascites fluid if the cells are transplanted into carrier mice) contains monoclonal antisera. The selection processes used to separate the initial clones can be targeted to identify specific clones, producing antisera with high affinities and low cross-reactivity to related compounds. The high specificity of monoclonal antisera can cause problems for some endocrine assays. Many hormones circulate in the blood as heterogeneous mixtures of multiple forms. Some of these forms are caused by genetic differences in patients, and others are related to metabolic precursors and degradation products of the hormone. Genetic differences cause some patients to produce variant forms of a hormone such as luteinizing hormone (LH). These genetic differences can cause marked variations in measurements made using assays with specific monoclonal antisera, compared with more uniform measurements made using assays with polyclonal antisera that cross-react with the multiple forms.15 Well-characterized monoclonal antisera can be mixed together to make an “engineered polyclonal antiserum” with improved sensitivity and specificity.16 Cross-reactivity with precursor forms of the analytes and with metabolic degradation products can cause major differences in assays. For example, cross-reactivity with six molecular forms of human chorionic gonadotropin (hCG) causes differences in hCG assays, and crossreactivity with metabolic fragments causes differences in parathyroid hormone (PTH) assays.17,18 Cortisol is another analyte for which major cross-reactivity with other ster­ oids, such as corticosterone, 11-deoxycortisol, cortisone, and numerous synthetic steroids, causes significant immunoassay interferences.19 Matrix effects with albumin also can cause major differences in cortisol immunoassays20 (see “Mass Spectrometry” for a more robust method for measuring steroids). Extraction of hormones from serum and urine specimens before measurement is a technique that can enhance both the sensitivity and the specificity of immunoassays. Numerous extraction systems have been developed, including organic-aqueous partitioning to remove water-soluble interferences seen with steroids, solid-phase extraction with absorption and selective elution from resins such as silica gels, and immunoaffinity chromatography.21,22 However, extraction and purification before immunoassay are seldom used in clinical assays; these techniques are difficult to automate and require skills and equipment not available in many clinical laboratories. Although

Laboratory Techniques for Recognition of Endocrine Disorders   87

Antisera binding site

Hormone

Biologic receptor binding site

Cultured cell in which binding of hormone to receptor activates cAMP ATP

Antisera cAMP

Hormone receptor

Cell

Figure 6-3  Comparison of an immunologic technique for measuring hormone concentration versus a receptor technique for measuring hor­ mone activity. ATP, Adenosine triphosphate; cAMP, cyclic adenosine monophosphate.

commercial assays generally use reagents having adequate sensitivity and specificity to measure most patient specimens, some patient specimens may give spurious results, and some disease states may require more analytic sensitivity to ensure sound clinical decisions. In these cases, extraction of specimens before measurement may provide more reliable information. Immunoassays measure concentrations rather than biologic activity. For most hormones, there is a strong correlation between the concentration of the protein or steroid being measured and the biologic activity, but this is not universally true. The reactive site for most antibodies is relatively small, about 5 to 10 amino acids for linear peptides. Some antiserum reactions are specific for the tertiary structure that corresponds to unique molecular configurations, but immunoassays seldom react with the exact antigenic structure that confers biologic activity. Figure 6-3 presents a schematic illustration of the dif­ ference between the immunologic binding site and the biologic receptor binding site on a hormone. Indirect immunoassays have been developed using cultured cells that synthesize second messengers such as cyclic adenosine monophosphate (cAMP) at rates related to the concentration of hormone in the specimen. An example of this technique is the immunoassay measurement of cAMP produced by osteosarcoma cells to quantitate PTH bioactivity in serum.23 Unfortunately, these assays are tedious and generally are not reproducible. Techniques using recombinant receptors as immunoassay binders may provide improved specificity with good reliability.24-26

to form a tertiary complex. As the antigen concentration increases, the signal increases progressively. Figure 6-4 schematically illustrates these concepts. The capture antiserum (ATB1) is attached to biotin (represented by solid circles). The signal antiserum (ATB2) is labeled with a detection system (asterisks). The ATB1-antigen-ATB2 complexes are immunologically extracted using a streptavidin solid phase (represented by the horizontal cups). After the complex is bound to the solid phase, most of the unbound signal antibody is washed away. As shown in Figure 6-4B, the signal increases progressively with the concentration. For lower concentrations, the signal increases proportionally to the assay concentrations (after the offset caused by the NSB). At higher concentrations of native antigen, the signal is generally less than proportional, so nonlinear curve-fitting techniques are used to generate the dose-response curves. Again, the relative imprecision, expressed as a coefficient of varia­ tion, depends on the slope of the dose-response curve; consequently, the relative precision is less at higher concentrations. In immunometric assays, the background level of signal is associated with very low concentrations. This background signal is caused by the NSB. The analytic sensitivity of immunometric assays is related to the ratio of the true signal to the NSB signal. Therefore, assays can be made more sensitive by increasing the response signal or by decreasing NSB. Inadvertent increases in NSB caused by

Capture ATB1

Signal ATB2

ATB1 + Ag + ATB2

Bound complexes

ATB1 • Ag • ATB2 + ATB1 + ATB2

100 100 100 100

0 4 12 36

100 100 100 100

0 4 12 36

Capture ATB1

Ag

Signal ATB2

Bound complex

100 96 88 64

100 96 88 64

Free ATBs

A 3500 3000

Immunometric (Sandwich) Assays

2500 Signal

A second immunologic technique used to measure hormones is the immunometric (sandwich) assay. The three basic components of a sandwich assay are (1) an antigen large enough to allow two antibodies to bind concurrently on different binding sites, (2) a capture antiserum directed to one of the antigenic sites on the antigen (this antiserum is attached to a solid phase to permit immunologic extraction of the immune complexes), and (3) a signal antiserum directed to a second antigenic site on the antigen (this antiserum is attached to an assay signal system). In contrast to competitive immunoassays, these assays use a large excess of antiserum-binding sites compared with the concentration of antigen. The capture antibody immunoextracts the antigen from the sample, and the signal antibody binds to the capture antibody–antigen complex

Ag

Conc. Signal

2000

0 4 12 36

1500 1000

(NSB) 500 1350 3100

500 NSB

0 0

B

10

20 Concentration

30

40

Figure 6-4  A, Principles of immunometric assays. Ag, antigen; ATB1, capture antiserum; ATB2, signal antiserum; ATBs, antibodies. See text for details. B, Typical dose-response curve. NSB, nonspecific binding level.

88   Laboratory Techniques for Recognition of Endocrine Disorders 100

Signal

80 60 40 20 0 0

10

100

1000

10,000 100,000 1,000,000

Hormone concentration, mIU/L Figure 6-5  Immunometric high-dose hook effect. The response signal reaches a maximum and then decreases when the antigen concentration exceeds the limit of the assay.

specimen interference or reagent deterioration can significantly alter assay performance. In immunometric assays, it is also important that a large excess of capture antibody be used. As the antigen concentration approaches the effective binding capacity of the capture antibody system, the signal no longer increases. If the antigen concentration exceeds the binding capacity of the capture antibody, the signal may actually decrease. Figure 6-5 illustrates this high-dose hook effect for immunometric assays caused by insufficient amounts of capture or signal antiserum.27 The signal increases progressively until the hormone concentration exceeds the binding capacity; the signal then decreases, probably as a result of removal of some of the more weakly binding antigenantibody complexes during the wash cycle on the assay.28-30 This is a potentially dangerous phenomenon, because the same values might be measured with very high and lower concentrations. If this artifact is suspected, the specimen may be diluted and reanalyzed. If the measured value for the diluted specimen is higher than the original result, a high-dose hook effect probably is present. Most manufacturers are aware of this potential problem and configure assays with relatively large amounts of capture antibody; however, some patients produce high concentrations of hormones or antigens that may exceed assay limits. Laboratories can detect this phenomenon by analyzing specimens at two dilutions, but this practice generally is not cost-effective. Therefore, feedback to the laboratory about results that are inconsistent with clinical findings is essential. Another potential problem for immunometric assays is the presence of endogenous heterophile antibodies that cross-react with reagent antiserum.31 Normally, the signal antibody does not form a “sandwich” with the capture antibody unless the specific antigen is present; however, divalent heterophile antibodies may mimic the antigen by simultaneously binding to the signal and capture reagent antibodies, thereby causing falsely elevated results.32,33 Figure 6-6 schematically illustrates this situation. The problem is most common with monoclonal antibodies but may also occur with polyclonal antibodies. Immunoglobulins contain both a constant (Fc) region and a variable (Fab) region. As implied in the name, the Fc region is constant, or similar, for all immunoglobulins from that species. Therefore, if a patient receives immunotherapy or imaging reagents containing mouse immunoglobulin, he or she is likely to develop human antimouse antibodies (HAMAs) directed to the Fc fragment.34 Some patients develop heterophile antibodies after exposure to foreign proteins from

domestic pets or food contaminants. If these endogenous antibodies are present in a patient’s specimen, they may bridge across the reagent antibodies used in immunometric assays and cause falsely high values. These antibodies also may bind to sites on the reagent antibodies, sterically blocking the binding of the specific antigen and leading to falsely low test values. Most manufacturers include nonimmune immunoglobulin in the assays to help block these interferences; as with the high-dose hook effect, however, the amounts added are not always adequate, and some patients with high-titer antibodies may still show in vitro assay interference.35 The combined specificity of the two antibodies used in an immunometric assay can produce exquisitely sensitive and specific immunoassays. In the past, a common problem with early competitive immunoassays was cross-reactivity among the structurally similar gonadotropins: LH, folliclestimulating hormone (FSH), TSH, and hCG. The α-subunits of each of these hormones are almost identical, and the β-subunits have considerable structural homology. Many individual antisera (especially polyclonal antisera) used for measuring one of these hormones may have cross-reactivity for the other gonadotropins. The cross-reactivity of a pair of antibodies is less than the cross-reactivity of each of the individual antibodies because any cross-reacting substance must contain both of the binding epitopes in order to simultaneously bind to both antibodies. For example, consider two antibodies for LH, each having 1% cross-reactivity with hCG. The cross-reactivity of the pair is less than the product of the two crossreactivities or, in this case, less than 0.01%. Most current immunoassays for LH have a cross-reactivity of less than 0.01%. This low cross-reactivity is important, because pregnant patients and patients with choriocarcinoma can have very high hCG concentrations that could interfere with measurements of the other gonadotropin hormones. Most hormones circulate in the blood in multiple forms. Some hormones (e.g., prolactin, growth hormone) circulate with macro forms, which can cause difficulty in their analysis if specimens are not pretreated.8 For hormones composed of subunits (e.g., the gonadotropins), both the intact and the free subunits circulate in blood. Immunometric assays can be made specific for intact molecules by pairing an antibody specific for the α-β bridge site of the subunits with a second antibody specific for the β-subunit. Assays using these antibody pairs retain the two-antibody, low cross-reactivity needed for measuring gonadotropins and do not react with the free subunit forms of the hormones.

False high Capture ATB1 Bridging ATB Signal ATB2 False low Capture ATB1 Blocking ATB Ag sterically blocked Figure 6-6  Assay interferences caused by heterophile antibodies, which result in either falsely high or falsely low values. Ag, antigen; ATB, antibody; ATB1, capture antiserum; ATB2, signal antiserum.

Laboratory Techniques for Recognition of Endocrine Disorders   89 TABLE 6-1 

Effect of Immunoassay Specificity on Calibration of Human Chorionic Gonadotropin (hCG) Assay Assay 1 Specificity for intact hCG standard (%) Cross-reactivity with free β-hCG (%) Measured values (IU/L)   Specimen with 0% free β-hCG   Specimen with 10% free β-hCG   Specimen with 50% free β-hCG

100 0 10.0 9.0 5.0

Assay 2 100 100 10.0 10.0 10.0

Assay 3 100 200 10.0 11.0 15.0

The heterogeneous forms of circulating hormones and differences in specificity characteristics of immunoassays for these forms make calibration and harmonization difficult. Two immunoassays calibrated with the same reference preparation can give widely varying measurements on patient specimens. Consider the example of hCG in Table 6-1. The three assays are calibrated with a pure preparation of intact hCG, such as the WHO Third International Reference Preparation.35 The three assays differ in their cross-reactivity with free β-hCG (0%, 100%, and 200%, respectively). These assays give identical measurements for a specimen containing only intact hCG but progressively disparate values as the percentage of free β-hCG in the specimen increases. In reality, the standardization issue is much more complex, because multiple forms of hormones (i.e., intact hormone, free subunits, nicked forms, glycosylated forms, degradation products) circulate in patients, and each assay has different cross-reactivities for these forms.17,36

Free (Unbound) Hormone Assays Many hormones are tightly bound to specific plasmabinding proteins and loosely bound to albumin. Usually, only the unbound (free) forms and some of the loosely bound forms are biologically active. Many methods are available to measure these free, unbound forms of a hormone. Theoretically, the best procedure is direct measurement of the free hormone concentration after physical separation of the free hormone from the bound hormone by equilibrium dialysis, ultrafiltration, or gel filtration. However, this method is difficult to perform and therefore is not readily available, and it is subject to technical errors. The two major clinical applications for free hormone measurements are for thyroid hormones (i.e., thyroxine [T4] and triiodothyronine [T3]) and for steroids (testosterone and estradiol). Four techniques are commonly used to estimate free thyroid hormone concentrations: indirect index methods, two-step labeled hormone methods, onestep labeled hormone analogue methods, and labeled antibody methods.

Indirect Index Methods The indirect indices involve two measurements: one for total hormone concentration and another for the thyroxinebinding globulin (TBG), followed by calculation of the ratio or of a normalized free thyroid hormone index (FT4I or FT3I). The availability of test results for total T4 and T4 binding capacity has the advantage of assessing these two different quantities but the disadvantage that ratios and indices are subject to the combined error of both

measurements. These methods correct for routine changes in TBG such as those associated with estrogen levels, but they may produce inappropriately abnormal values in patients with extreme variations in TBG levels, such as those with congenital disorders of the TBG gene, familial dysalbuminemic hyperthyroxinemia, thyroid hormone autoantibodies, or certain nonthyroidal illnesses. Because of the necessity for two measurements and the sensitivity of these methods to interference with drugs, these indirect methods are being used less frequently.

Two-Step Labeled Hormone Methods The two-step labeled hormone methods immunologically bind the free and loosely bound thyroid hormone to a solid phase. The other serum components are washed away, and the residual binding sites are back-titrated with labeled hormone. When calibrated with appropriate serum standards, these methods are thought to pose fewer problems related to binding protein abnormalities.

One-Step Labeled Hormone Analogue Methods The one-step labeled hormone analogue methods use synthetic analogues of T4 and T3 that bind to the measurement antibody but do not bind to normal TBG. These methods are seldom used because performance has been poor in patients with abnormal albumin concentrations, abnormal free fatty acid concentrations, and all conditions that interfere with the indirect indices.37

Labeled Antibody Methods The labeled antibody methods use kinetic reactions of antibodies with selected affinities that bind preferentially with the free form of the hormone. These methods work best for automated testing instruments and have become popular.

Complexities in Testing Each of the methods described here works well for correcting for minor changes in TBG levels, but each has problems with some patient sera, especially sera containing interfering substances such as inhibitors and heterophilic antibodies. Most manufacturers have not fully validated their methods in patients with these abnormalities.38 Multiple methods are also available for measuring both the free and the biologically active forms of steroid hormones. The preferred method for measurement of free hormones consists of direct physical separation and high-sensitivity assays, similar to those methods recommended for the thyroid hormones. One-step labeled hormone analogue methods also have been developed, but they are associated with interference problems similar to the problems associated with free thyroid hormone assays. The measurement of free testosterone has been problematic. Most immunoassays are unreliable at low concentrations, and the concentration of free testosterone is much lower than that of total testosterone. Another complexity in regard to steroid hormones is that, in addition to the free hormones, testosterone and estrogen bound to albumin also are biologically active. The concentration of the biologically active forms can be estimated with the use of indirect indices calculated from measurements of the total hormones and sex hormonebinding globulin (SHBG) or by measurement of the residual free and albumin-bound steroids after separation of the SHBG-bound forms after differential precipitation with ammonium sulfate. Recent work with tandem mass spectrometry shows promise as a more reliable test method (see later discussions).

90   Laboratory Techniques for Recognition of Endocrine Disorders Quadrupole Mass Spectrometer

Chromatographic Assays The second major method of measuring hormone concentrations involves chromatographic separation of the various biochemical forms and quantitation of specific characteristics of the molecules. High-performance liquid chromatography (HPLC) systems use multiple forms of detection, including light absorption, fluorescence, and electrochemical properties.39,40 Chromatography also is frequently combined with mass spectrometry. There are two major advantages of these techniques: they can be used to simultaneously measure multiple forms of an analyte, and they are not dependent on unique immunologic reagents. Therefore, harmonization of measurements made with different assays is more feasible. The major disadvantages of these methods are their complexity and their limited availability. Many chemical separation techniques are based on chromatography, but the two most commonly used for liquid chromatography are normal-phase HPLC and reversephase HPLC.29 In both systems, a bonded solid-phase column is made that interacts with the analytes as they flow past in a liquid solvent. In normal-phase HPLC, the functional groups of the stationary phase are polar (e.g., amino or nitrile ions) relative to the nonpolar stationary phase (e.g., hexane); in reverse-phase HPLC, a nonpolar stationary phase (e.g., C18 octadecylsilane molecules bonded to silica) is used. More recently, polymeric packings made of mixed co­ polymers have been made with C4, C8, and C18 functional groups directly incorporated so that they are more stable over a wide pH range. The mobile and stationary phases are selected to optimize adherence of the analytes to the stationary phase. The adhered molecules can be eluted differentially from the solid phase, after washing to separate specific forms of the analyte from interfering substances. If the composition of the mobile phase remains constant throughout the run, the process is called an isocratic elution. If the mobile-phase composition is abruptly changed, a step elution occurs. If the composition is gradually changed throughout the run, a gradient elution occurs. The efficiency of separation in a chromatography system is a function of the flow rates of the different substances.41 The resolution of the system is a measure of the separation of the two solute bands in terms of their relative retention volumes (Vr) and their bandwidths (ω). Resolution (Rs) of solutes A and B is shown as Rs =

2 [ Vr ( B) − Vr ( A )] ω ( A ) + ω (B)

Values of Rs lower than 0.8 result in inadequate separation, and values greater than 1.25 correspond to baseline separation. The resolution of a chromatography column is a function of flow rates and thermodynamic factors. Simultaneous measurement of the three catecholamines (epinephrine, norepinephrine, and dopamine) can be performed with reverse-phase HPLC with a C18 column and an electrochemical detection system42 or fluorometric detection.43 Prior extraction by absorption on activated alumina and acid elution helps improve specificity. Dihydroxybenzylamine, a molecule similar to endogenous catecholamines, can be used as an internal standard.

Mass Spectrometry The technique of mass spectrometry involves fragmentation of target molecules, followed by separation and measurement of the mass-to-charge ratio of the components.41

Quadrupole ion analyzer + + Ionizer Develop charged particles

Detector Ion selection

Quantitation of relative abundance of mass/charge entities

Figure 6-7 Basic components of quadrupole mass spectrometer.

When coupled with liquid chromatography, a mass spectrometer can function as a unique detector to provide structural information about the composition of individual solutes.44 Inclusion in the specimens of internal standards, which are molecularly similar to the measured compounds, allows precise quantitation of the concentration of the eluting analytes. The measurement of specific mass fragments makes possible the quantitation of multiple specific analytes in complex mixtures. The basic components of a quadrupole mass spectrometer are an ionizer, an ion analyzer, and a detector (Fig. 6-7). The initial step in mass spectrometry is fragmentation of the target compound into charged ions. Many techniques are used to generate these charged ions, including chemical ionization and electron-impact ionization. Chemical ionization uses reagent gas molecules such as methane, ammonia, water, and isobutane to transfer protons. This process produces less fragmentation than other techniques because the process is not highly excited. The electron impact method bombards gas molecules from the sample with electrons emitted from a heated filament. The process occurs in a vacuum to prevent the filament from burning out. Electron-spray ionization is a process in which a solution containing the analyte is introduced into a gas phase and is sprayed across an ionizing potential.45 The charged droplets are desolvinated and analyzed in a mass spectrometer. The ion analyzer uses four charged rods to systematically set up a charged field that selects only certain ions with particular mass-to-charge ratios and facilitates their movement along a path to the detector. Regular calibration is necessary to ensure accuracy of the instrument. A mass spectrum is a bar graph in which the heights of the bars correspond to the relative abundance of a particular ion plotted as a function of the mass-to-charge ratio. Modern mass spectrometers can measure molecular masses so accurately and precisely that the elemental composition of a compound can be predicted by comparison with stored spectral libraries. When these systems are used to measure only a few selected compounds with known spectrums, the mass spectrometer can be programmed to focus only on these selected ions. Stable isotopes of the compounds of interest can be used as internal standards through a technique called isotope dilution mass spectrometry. Stable isotopes typically perform the same as the native compounds in terms of extraction, chromatography, and mass spectrometry and therefore are ideal internal standards. However, there must be a sufficient number of isotopic atoms to ensure that their mass is different from that of naturally occurring substances that may be in the specimen.

Laboratory Techniques for Recognition of Endocrine Disorders   91 7.46

100

DHEAS

0

9.56

100

Cortisol

0

11.08

100

11-Deoxycortisol

0

11.88

11.07

100

Androstenedione

Percent

0 100

12.67

7.48

Estradiol

0 100

Testosterone

13.22

0 100

13.98

17-Hydroxyprogesterone

0 100

DHEA

7.43

13.99

0

16.85 Progesterone

100 0 1

2

3

4

5

6

7

8

9

10

11 12

13

14

15

16

17

18

Time (min) Figure 6-8  Liquid chromatography–tandem mass spectroscopy profiles of nine steroids.49 DHEAS, Dehydroepiandrosterone 3-sulfate; DHEA, dehydroepiandrosterone. (From Guo T, Chan M, Soldin SJ. Steroid profiles using liquid chromatography-tandem mass spectrometry with atmospheric pressure photoionization source. Arch Pathol Lab Med. 2004;128:469-475. Reproduced with permission from Archives of Pathology of Laboratory Medicine.)

Tandem mass spectrometry (MS/MS) is a powerful tool consisting of two mass analyzers separated by an ionactivation device.46,47 The first analyzer is used to isolate and dissociate the ion of interest by activation, and the second mass analyzer to analyze its dissociation products. This technique can be used to provide rapid, definitive measurements of multiple endocrine analytes.44 For example, liquid chromatography and tandem mass spectrometry can be used to simultaneously quantitate multiple steroid compounds.48-52 In Figure 6-8, the chromatograph shows well-separated peaks for nine steroids in a standard solution.48 Standardization and harmonization of hormone assays have become priorities for quality health care. The National Institute of Standards and Technology (NIST) has developed mass spectrometry reference methods for measuring cortisol, progesterone, estradiol-17β, T4, and T3.51,53-58 The International Federation of Clinical Chemistry (IFCC) has proposed reference measurement procedures for free T4.59,60 MS methods also have been developed for insulin, PTH, 17-hydroxyprogesterone, and free T3, although these are not considered reference methods at this time. Efforts are under way to try to harmonize clinical measurements using these reference methods; however, that will be a dif­ficult task because of the difference inherent in the antibody specificity of the immunoassays used for most measurements.

Nucleic Acid–Based Assays The decoding of the human genome has set the stage for a large potential increase in nucleic acid–based gene assays. The basic principles of nucleic acid–based assays have been known for several decades, but the identification of specific

genes and the mapping of gene defects to clinical disease states have now made these measurements clinically useful.61 However, as this field has evolved, it has been found that genetic heterogeneity is very common and clinical testing usually is based on strategies to find mutations as efficiently as possible.62 Four concepts important for nucleic acid measurements are hybridization, amplification, restriction fragment length polymorphisms (RFLPs), and electrophoretic separation.63 Newer techniques such as comparative genomic hybridization (CGH) and nextgeneration sequencing offer potential advances for genomic testing.64-66

Hybridization Nucleic acid molecules have a unique ability to fuse with complementary base-pair sequences. When a fragment of a known sequence (probe) is mixed under specific conditions with a specimen containing a complementary sequence, hybridization occurs. This feature is analogous to the antibody-antigen binding used in immunoassays. Many of the formats used for immunoassay have been adopted to nucleic acid assays, including some of the same signal systems (e.g., radioactivity, fluorescence, chemiluminescence) and the same solid-phase capture systems (e.g., magnetic beads, biotin-streptavidin binding). In situ hybridization, which involves the binding of probes to intact tissue and cells, provides information about morphologic localization analogous to that provided by immunohistochemistry.

Amplification Nucleic acid assays have an advantage in that low concentrations can be amplified in vitro before quantitation. The best-known amplification procedure is the polymerase

92   Laboratory Techniques for Recognition of Endocrine Disorders chain reaction (PCR), first reported by Mullis and Faloona.67 The three steps in the process (denaturation, annealing, and elongation) occur rapidly at different temperatures. Each cycle of amplification can occur in less than 90 seconds by cycling the temperature. The target doublestranded DNA is denatured at high temperature to make two single-stranded DNA fragments. Oligonucleotide primers, which are specific for the target region, are annealed to the DNA when the temperature is lowered. Addition of DNA polymerase allows the primer DNA to extend across the amplification region, thus doubling the number of DNA copies. At 85% to 90% efficiency, this process can amplify the DNA by about 250,000-fold in 20 cycles. This huge amplification is subject to major problems with contamination if special precautions are not taken. In one control technique, a psoralen derivative is used to prevent subsequent copying by polymerase during exposure to ultraviolet light.

Restriction Fragment Length Polymorphisms Some diseases (e.g., sickle cell anemia) are associated with a specific gene mutation; usually, however, a series of deletions and additions of DNA are involved with a particular disease. A number of restriction enzymes that cleave DNA at specific locations have been identified. Changes in the sequence of DNA result in different fragment lengths. The RFLP technique is particularly helpful in family studies for disorders that have a unique genetic fingerprint.

Electrophoretic Separation E. M. Southern invented an electrophoretic separation technique known as Southern blotting.68 Restriction enzymes are used to digest a sample of DNA into fragments, and the product is subjected to electrophoresis. The separated bands of DNA are then transferred to a solid support and hybridized. Northern blotting is a similar technique in which RNA is used as the starting material. Western blotting refers to electrophoresis and transfer of proteins.

Newer Nucleic Acid Measurement Technologies Comparative Genome Hybridization CGH is a molecular-cytogenetic method for analyzing the gains and losses of a given subject’s DNA.64 This technique detects only unbalanced chromosome changes; balanced reciprocal translocations or inversions may not be detected. Currently, this procedure is an adjunct to standard karyotype analysis, but it may provide higher resolution.

Next-Generation Sequencing Next-generation sequencing uses large numbers of parallel processors to sequence clonally amplified DNA molecules separated in a flow cell.66 This provides advantages of speed, lower cost, and better accuracy. On the other hand, this technique can rapidly generate large amounts of sequence data, posing challenges to bioinformatic analysis. The interpretation of these data requires linkage with large databases and a good understanding of genomic variations in the targeted regions.

ANALYTIC VALIDATION Clinicians usually assume that laboratory methods have been validated and that they function correctly. Although this assumption is generally true, it is helpful to

understand the level of assay validation performed and the appropriateness of the validation criteria for each clinical application of a test.69,70 In the United States, the federal government regulates all laboratories performing complex tests for patients receiving Medicare.71,72 These regulations, published in the Federal Register, outline the validation requirements for Food and Drug Administration (FDA)-approved instruments, kits, and test systems as well as methods developed in house. Laboratories must document analytic accuracy, precision, reportable ranges, and reference ranges for all procedures. The regulations for in-house procedures and modifications of approved commercial procedures are more extensive and require laboratories to further document the analytic sensitivity; analytic specificity, including interfering substances; and other performance characteristics required for testing patient specimens. Although the details of method validation may be unique to a specific procedure, the following analytic validation studies have proved valuable for most procedures: method comparison, precision, linearity, recovery, detection limit, reportable range, analytic interference, carryover, reference interval, specimen stability, and specimen type. Similar recommendations for validating nucleic acid tests were recently published by the College of American Pathologists.73 Laboratories should have documentation for each of these performance characteristics, either from the diagnostics manufacturer or from direct studies.

Method Comparison Ideally, the system should be compared with an established reference method; however, many endocrine tests do not have reference methods, and many laboratories do not have the facilities to perform reference methods when they exist. At a minimum, the assay should be compared with an analytic system that has been clinically validated with specimens from healthy subjects and specimens from patients with the diseases being investigated.74 The system should be traceable to established reference standards, such as those from the WHO or the NIST.75-77 Between 100 and 200 different specimens distributed over the assay range are recommended for method comparisons.78 A cross-plot portraying the new method on the vertical axis and the established method on the horizontal axis, along with the identity line, reference value lines, and regression statistics, is a useful way of displaying these comparisons. An alternative display method is the BlandAltman difference plot, in which the difference between the test method and the reference method is plotted against the reference method values. Although acceptable performance criteria for method comparisons are not well established, some important characteristics to examine are • Any grossly discordant test values • The degree of scatter about the regression curve • The size of the regression offset on the vertical axis • The number of points crossing between the low, normal, and high reference intervals for the two methods The European Union has enacted an In Vitro Diagnostics Directive that requires manufacturers marketing in the European Union to establish that their products are “traceable to reference standards and reference procedures of a higher order” when such references exist. Hopefully, medically relevant performance characteristics that define the allowable ranges for differences between a specific assay’s test values and the traceable standards will be linked with

Laboratory Techniques for Recognition of Endocrine Disorders   93

this traceability requirement.79 This combination of traceability and allowable error requirements could serve to harmonize many test methods worldwide, because most diagnostic companies market internationally.80

TABLE 6-2 

Recommended Analytic Performance Limits* Analyte

Precision Precision is a measure of the replication of repeated measurements of the same specimen; it is a function of the time between repeats and the concentration of the analyte. Both short-term precision (within a run or within a day) and long-term precision (across calibrations and across batches of reagents) should be documented at clinically appropriate concentration levels.81 In general, normal-range, abnormally low-range, and abnormally high-range targets are chosen for precision studies; however, targets focused on critical medical decision limits may be more appropriate for some analytes. Twenty measurements are recommended at each level for both short-term and long-term precision validations. Precision usually is expressed as the coefficient of variation, calculated as 100 times the standard deviation (SD) divided by the average of the replicate measurements.82 There is no universal agreement on the performance criteria for analytic precision, although numerous recommendations have been put forth. Two major approaches to defining these criteria have been (1) comparison with biologic variation and (2) expert opinion of clinicians based on their perceived impact of laboratory variation on clinical decisions. The total variation clinically observed in test measurements is a combination of the analytic and biologic variations. For instance, if the analytic SD is less than one fourth of the biologic SD, the analytic component increases the SD of the total error by less than 3%. If the analytic precision is less than one half of the biologic SD, the total error increases by only 12%. These observations have led to recommendations for maintaining precision of less than one fourth or one half of the biologic variation. The expert opinion precision recommendations are based on estimates of the magnitude of change of a test value that would cause clinicians to alter their clinical decisions. Table 6-2 lists some precision recommendations for selected endocrine tests.83-85

Calcium Glucose Thyroxine Potassium Triiodothyronine Thyrotropin Cortisol Estradiol Follicle-stimulating hormone Luteinizing hormone Prolactin Testosterone Insulin Dehydroepiandrosterone 11-Deoxycortisol

Biologic CVi (%)

Precision (%)

Accuracy (%)

1.8 4.4 7.6 4.4 8.7† 20.2† 15.2† 21.7† 30.8† 14.5† 40.5† 8.3† 15.2† 5.6† 21.3†

0.9† 2.2† 3.4† 2.4† 4.0‡ 8.1‡ (7.6)* (10.9)* (15.4)* (7.2)* (20.2)* (4.1)* (7.6)* (2.8)* (10.6)*

0.7 1.9 4.1 1.6 5.5‡ 8.9‡

*Numbers in parentheses correspond to one half of within-individual coefficient of variation. † Data from Fraser CG. Biological variation in clinical chemistry: an update— collated data 1988-1991. Arch Pathol Lab Med. 1992;116:916-923. ‡ Data from Fraser CG, Petersen PH, Ricos C, et al. Proposed quality specifications for the imprecision and inaccuracy of analytical systems for clinical chemistry. Eur J Clin Chem Clin Biochem. 1992;30:311-317. CVi, intraindividual variation. Data from Stockl D, Baadenhuijsen H, Fraser CG, et al. Desirable routine analytical goals for quantities assayed in serum. Discussion paper from the members of the External Quality Assessment (EQA) Working Group A on analytical goals in laboratory medicine. Eur J Clin Chem Clin Biochem. 1995;33: 157-169.

low-concentration specimens. Some analytes circulate in the blood in multiple forms, and some of these forms may be bound to carrier proteins. The recovery rate of pure substances added to a specimen may be low if the assay does not measure some of the bound forms. Mixtures of patient specimens may not be measured correctly if one of the specimens contains cross-reacting substances such as autoantibodies. A thorough understanding of the chemical forms of the analyte and their cross-reactivities in the assay is important during assessment of recovery data.

Linearity

Detection Limit

Patient specimens commonly contain several different forms of the hormones to be measured, whereas pure forms are contained in the reference standards and calibrators used to establish the assay dose-response curve.86 When a patient specimen is diluted, the measured value for these dilutions should parallel the dose-response curve and give results proportional to the dilution. Linearity can be evaluated by measuring serial dilutions of patient specimens, with high concentrations diluted in the appropriate assay diluents.87,88 The product of the measured value multiplied by the dilution factor should be approximately constant. There are no performance standards for linearity, but a reasonable expectation for most hormones is that dilutions are comparable within 10% of the undiluted value.

The minimal analytic detection limit is the smallest concentration that can be statistically differentiated from zero. This concentration is mathematically determined as the upper 95% limit of replicate measurements of the zero standard, calculated from the average signal plus 2.0 SD. This minimal detection limit is valid only for the average of multiple replicate measurements. When individual determinations are performed on a specimen having a true concentration exactly at the minimal detection limit, the probability that the measurement is above the noise level of the assay is only about 50%. A second parameter for the lowest level of reliable measurement for an assay is the functional detection limit, or the limit of quantitation. For this value to be measured, multiple pools with low concentrations are made and analyzed in the replicate. A cross-plot of the coefficient of variation of the measurements versus concentration allows one to generate a precise profile. The concentration corresponding to a coefficient of variation of 20% is the functional detection limit.1 This term typically applies to across-assay variation, but it also can be calculated for within-assay variation if

Recovery Two methods of assessing the recovery of assays are (1) measuring the increase in test values after the reference analyte is added and (2) measuring the proportional changes caused by mixing high-concentration and

94   Laboratory Techniques for Recognition of Endocrine Disorders one uses the tests to evaluate results measured within one run (e.g., provocative and suppression tests).

Reportable Range The reportable range of an assay usually spans from the functional detection limit to the concentration of the highest standard. Values above the highest standard may be reported if they are diluted and the measured value is multiplied by the dilution factor. The validity of the analytic range is documented by the linearity and recovery studies. Some laboratories erroneously report the exact values displayed by the test systems even if they are outside the analytic range. It is important for clinicians to understand the limitations of valid measurements and not to inappropriately use meaningless numbers that may be reported. Another potential source of error is failure of the technologist to multiply the measured value of diluted specimens by the dilution factor to correct for the dilution. In addition, care should be taken to define the number of significant figures used for reporting test values and to establish an appropriate algorithm for rounding test values to the significant number of digits.

Analytic Interference The cross-reactivity and potential interference of other analytes that may react in a test system should be documented.84 The choice of potential interfering substances that must be evaluated requires an understanding of the analytic system and the pathophysiology of the analyte being evaluated. In immunoassays, for example, compounds with similar structures, as well as precursor forms and degradation products, should be tested.89-92 Drugs commonly prescribed for the diseases under evaluation should be assessed for interference, both by addition of the drug to a specimen and by analysis of specimens from patients before and after receiving the drug.93,94 Most assays also are evaluated for the effects of hemolysis, lipemia, and icterus.

Carryover Studies Many diagnostic systems use automated sample-handling devices. If a specimen to be tested is preceded by a specimen with a very high concentration, a trace amount remaining from the first specimen may significantly increase the reported concentration in the second specimen. The choice of the concentration that should be tested for carryover depends on the pathophysiology of the disease, but high values may need to be tested because some endocrine disorders can produce high values. A prudent procedure would be to retest all specimens after a specimen with an extraordinarily high value. One also should document that carryover from the sampling probe has not inadvertently contaminated subsequent specimen vials, thereby invalidating subsequently repeated measurements.

Reference Intervals The development and validation of reference intervals for endocrine tests can be very complex tasks.95,96 The normal reference interval for most laboratory tests is based on estimates of the central 95-percentile limits of measurements in healthy subjects.97 A minimum of 120 subjects is needed to reliably define the 2.5 and 97.5 percentiles. The reference intervals for many endocrine tests depend on

gender, age, developmental status, and other test values. Formal statistical consultation is recommended to determine the appropriate number of subjects to test and to develop statistical models for defining multivariate reference ranges. Full evaluation of the adrenal, gonadal, and thyroid axes requires simultaneous measurement of the trophic and target hormones. Bivariate displays of these hormone concentrations along with their multivariate reference intervals facilitate the interpretation.98 Preanalytic conditions should be well defined and controlled during evaluation of both healthy reference subjects and patients.

Specimen Stability Analyte stability is a function of storage conditions and specimen type.99 Although most hormones are relatively stable in serum or urine if they are rapidly frozen and stored in hermetically sealed vials at −70° C, multiple freeze/thaw cycles can damage analytes, and storage in frost-free freezers that repeatedly cycle through thawing temperatures can adversely affect stability. Blood specimens collected in edetate (EDTA) often are more stable than serum or heparinized specimens because edetate chelates calcium and magnesium ions that function as coenzymes for some proteases. The addition of protease inhibitors (e.g., aprotinin) to blood specimens may also improve specimen stability.100

Types of Specimens Most hormones are measured in blood or urine, but alternative testing sources, such as saliva and transdermal membrane monitors, are also used.

Urine Specimens The 24-hour urine specimen is used for many endocrine tests. Such urine specimens represent a time average that integrates over the multiple pulsatile spikes of hormone secretion occurring throughout the day. The 24-hour urine specimen also has the advantage of better analytic sensitivity for some hormones.101,102 Urine often contains not only the original hormone but also key metabolites that may or may not have biologic activity. Drawbacks include the inconvenience of collecting the 24-hour specimen and delays in collection. Another limitation of urine specimens is uncertainty regarding the completeness of the collection. Measurement of urinary creatinine concentrations helps in monitoring collection completeness, especially when this value is compared with the patient’s muscle mass. Many urinary hormones are conjugated to carrier proteins before excretion. Therefore, both hepatic function and, to a lesser degree, renal function may alter urinary hormone values.

Blood Specimens Blood specimens have both the advantage and the limitation of time dependency. The ability to detect rapid changes to a provocative stimulus is a strong advantage, whereas the unsuspected changes resulting from pulsatile secretions may be a major limitation. Most hormones undergo significant biologic variations, including ultradian, diurnal, menstrual, and seasonal changes.103-105 Many hormones have short half-lives and are rapidly cleared from the blood. The half-life is particularly important when one is attempting to measure the response to a provocative drug, such as the effect of gonadotropin-releasing hormone.106 The development of rapid intraoperative methods for

Laboratory Techniques for Recognition of Endocrine Disorders   95

measuring PTH and growth hormone has highlighted the importance of plasma specimens, which do not require extra waiting time for the blood to clot to make serum.107,108

Saliva Specimens Saliva is becoming an alternative specimen for measuring non–protein-bound hormones and small molecules.109-113 Small analytes in blood pass into oral fluid by crossing capillary walls and basement membranes and by passing through lipophilic membranes of epithelial cells.111 This transport involves passive diffusion, ultrafiltration, active transport, or some combination of these processes. The concentration in saliva depends on the concentration of the non–protein-bound analyte in blood, the salivary pH, the acid dissociation constant (pKa) of the analyte, and the size of the analyte. Analytes entering saliva by passive diffusion usually are less than 500 d in size, non–protein bound, and nonionized. Saliva measurements correlate with blood measurements for some hormones such as cortisol, progesterone, estradiol, and testosterone, but they do not correlate well for others (e.g., thyroid and pituitary hormones).114-116 Multiple preanalytic variables can affect the salivary measurement. Stimulation of oral fluid production by chewing or by the use of candy or drops that contain stimulants such as citric acid can increase oral fluid volume and stabilize pH but may alter some analyte concentrations. Several commercial devices are available for collection of oral fluid; however, these devices need to be validated for each analyte and each assay system to ensure they adequately recover each of the analytes.

Blood Drops Blood drops collected on filter paper from punctures of a finger or heel are a convenient system for collecting, transporting, and measuring hormones.117,118 If standardized collection conditions and extraction techniques are used, these measurements correlate well with serum measurements. Integration of immunochemistry with computer chip technology has also led to immunochips that can measure multiple analytes using a single drop of blood.119

Noninvasive Measurements Noninvasive transcutaneous measurements also have been developed for some endocrine tests.120 Transcutaneous glucose measurements using near-infrared spectroscopy correlate well with blood measurements.121 Glycemic control in subjects with diabetes requiring insulin has been shown to improve with continuous real-time transcutaneous glucose monitoring.122

the recent past, but they provide little assurance that measurements are adequate for clinical decisions. Statistically, there are two major forms of analytic errors: random and systematic. Random error relates to reproducibility; systematic error relates to the offset or bias of the test values from the target or reference value. Performance criteria can be defined for each of these parameters, and quality control systems can be programmed to monitor compliance with these criteria. Control systems must have low false-positive rates as well as high statistical power to detect assay deviations. The multirule algorithms developed by Westgard and colleagues126 use combinations of control rules—such as two consecutive controls outside of warning limits, one control outside of action limits, or moving average trend analyzers outside of limits—to achieve good statistical error detection characteristics.127 Traditionally, quality control programs have focused primarily on precision; however, analytic bias also can cause major clinical problems. If fixed decision levels are used to trigger clinical actions (e.g., therapy, additional investigations), changes in the analytic set point of an assay can cause major changes in the number of follow-up cases.127 This concept is illustrated in Figure 6-9 for TSH measurements. Under stable laboratory testing conditions, approximately 122 per 1000 patients tested have TSH values greater than 5.0 mIU/L. If the test shifts upward by 20%, the number of patients with TSH values greater than 5.0 mIU/L increases to 189, which is an increase of more than 50% in the number of patients flagged as abnormal (see Fig. 6-9). These changes in test value distributions often can be sensed by clinicians who encounter multiple patients with unexpectedly elevated test values, causing them to call the laboratory and inquire whether the “test is running high today.” Some modern quality control systems use moving averages of patient test values to help monitor changes in analytic bias.128 Some medical facilities have linked together into networks to provide more integrated patient care. This crossover of both physicians and patients is increasing the importance of harmonized testing systems. For endocrine tests, harmonization is best achieved when all the laboratories in the network use the same test systems. Differences in analytic specificity may cause across-method differences in patient test distributions even when the methods use the same reference standards. Full harmonization of testing requires not only standardization of equipment but also standardization of reagents (including use of the same lot numbers) and standardization of laboratory protocols.

800

QUALITY ASSURANCE

Number per 1000 > 5 mlU/L

700 600

Laboratory quality control programs are intended to ensure that the test procedures are being performed within defined limits. A critical component of control systems is the definition of acceptable performance criteria.123,124 Such criteria often are not well defined, and many laboratories use floating criteria that change when assays change.125 Control limits are often set at the mean ± 2 or 3 SDs, where the mean and the SD are arbitrarily assigned based on measurements made in that laboratory. When reagents or equipment changes, new limits are assigned. These types of control systems provide some assurance that the laboratory is functioning at a level of performance similar to that of

500

Counts

Quality Control Systems

Original = 122 After shift = 189

51%

400 300 200 100 0 0

1

2

3

4 5 6 TSH, mlU/L

7

8

9

10

Figure 6-9  Effect of analytic bias, or shift, on the number of patients with elevated levels of thyrotropin (TSH).

96   Laboratory Techniques for Recognition of Endocrine Disorders Real-time quality control monitors with peer group comparisons across the laboratories in the health care network are necessary to ensure uniformity of testing.

100 80 70

D = X1 − X2 where X1 is the first measurement, X2 is the repeated measurement, and D is the difference. Variance (D) = Variance (X1 ) + Variance (X2 ) Variance (D) = 2 Variance (X ) SD (D) = 2 Variance (X ) SD (D) = 2SD (X ) The variance of D is the sum of the variance of X1 and the variance of X2. The SD of D is the square root of the variance of D, or the square root of twice the variance of X1. The SD of D equates to the square root of 2 multiplied by SD(X). Therefore, 95% of the absolute values for D should be within square root of 2 times 2 SD(X), or approximately 3 SD(X). If a repeat measurement exceeds this 3 SD(X) limit, the initial (or reagent) measurement is probably in error. Linearity and recovery are valuable techniques for evaluating test validity. If the initial test value is elevated, serial dilution of the specimen in the assay diluent and reassay should be considered. If the specimen dilutes nonproportionately, no meaningful value can be reported with that assay. In the example in Figure 6-10, the undiluted specimen reads 22, the twofold dilution multiplies back to 34 (2 × 17), and the fourfold dilution multiplies back to 60 (4 × 15). Therefore, the result depends on the dilution factor, so that no reliable answer can be reported. If the initial value is low, one may consider adding known quantities of the analyte to part of the specimen. Analyzing these spiked or diluted specimens with the original specimen allows one to evaluate both reproducibility and recovery. It may be helpful to analyze the linearity or recovery of the assay standards at the same time, to provide internal controls of the dilution or spiking procedures and the appropriateness of the diluent and spiking material. If the replication, dilution, or recovery experiment appears successful, further analytic troubleshooting will

Signal

Investigation of Discordant Test Values The practice of modern endocrinology depends extensively on reliable and accurate test values; even in the best laboratories, however, erroneous results sometimes are reported. Careful correlation of pathophysiology with test values can help to identify values that are “discordant.”98 Some of these discordant test values may be analytically correct, but others may be erroneous. Clinicians can help investigate these suspicious test values by requesting laboratories to perform a few simple validation procedures. Repeated testing of the same specimen is a valuable first step. If the specimen has been stored under stable conditions, the absolute value of the difference between the initial and the repeated measurements should be less than 3 analytic SDs 95% of the time. Normally, the 95% confidence range is associated with the mean ± 2 SDs; with repeated laboratory tests, however, errors are associated with the first as well as the second measurement. The confidence interval for the uncertainty of the difference between two measurements can be calculated using the statistical rules for propagation of errors. To better understand this propagation of error, consider

Standard

90

60 50 40 30

X 1:4 = 60

20 10 0 0

10

(15)(17) (22) 20

X 1:2 = 34

30

X Patient Nt = 22

40

50

Concentration Figure 6-10  Nonproportional dilutions. Discordant values are produced when samples do not dilute linearly. Nt, undiluted (neat).

vary according to the method used. Immunoassays may be affected by interference caused by heterophile antibo­ dies. Addition of nonimmune mouse serum or heterophile antibody-blocking solutions may neutralize these effects.129,130 Chromatographic assays are usually more robust than immunoassays. Specimens with suspected interference on one type of assay can be reanalyzed by means of an alternative methodology. Water-soluble interferences have been reported for some direct assays for steroid measurements.21,22,131 Extraction of the hormones into organic solvents, followed by drying down and reconstitution in the assay zero standard, removes these interferences. Similarly, interferences with cross-reacting drugs and metabolic products can be minimized with selective extraction. The analytic methods of assessing endocrine problems in patients are continually expanding. The newer systems are often based on analytic techniques similar to those outlined in this chapter, but the configurations are generally more user-friendly. These advances make the systems more convenient, but they also become more of a “black box” that conceals most of the details of the system. The performance validation steps outlined in this chapter become important procedures for ensuring that these systems continue to provide the reliable measurements needed for quality medical care. REFERENCES 1. Spencer CA, LoPresti JS, Patel A, et al. Applications of a new chemiluminometric thyrotropin assay to subnormal measurement. J Clin Endocrinol Metab. 1990;70:453-460. 2. Klee GG, Hay ID. Biochemical testing of thyroid function. Endocrinol Metab Clin North Am. 1997;26:763-775. 3. Price CP, Newman DJ, eds. Principles and Practice of Immunoassay. New York, NY: Stockton Press; 1996. 4. Thorell JI, Larson SM. Radioimmunoassay and Related Techniques: Methodology and Clinical Applications. St. Louis, MO: Mosby; 1978. 5. Rodbard D. Data processing for radioimmunoassays: an overview. In: Natelson S, Pesce AJ, Dietz AA, eds. Clinical Chemistry and Immunochemistry (AACC): Chemical and Cellular Bases and Applications in Disease. Washington, DC: AACC; 1978:477-494. 6. Fomenko I, Durst M, Balaban D. Robust regression for high throughput drug screening. Comput Methods Programs Biomed. 2006;82:31-37. 7. Gosling JP. A decade of development in immunoassay methodology. Clin Chem. 1990;36:1408-1427. 8. Vieira JG, Tachibana TT, Obara LH, et al. Extensive experience and validation of polyethylene glycol precipitation as a screening method for macroprolactinemia. Clin Chem. 1998;44:1758-1759. 9. Klee GG, Post G. Effect of counting errors on immunoassay precision. Clin Chem. 1989;35:1362-1366.

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98   Laboratory Techniques for Recognition of Endocrine Disorders Molecular Diagnostics. 4th ed. Philadelphia, PA: Elsevier; 2006: 1407-1449. 64. Edelmann L, Hirschhorn K. Clinical utility of array CGH for the detection of chromosomal imbalances associated with mental retardation and multiple congenital anomalies. Ann N Y Acad Sci. 2009;1151: 157-166. 65. Shen Y, Wu BL. Microarray-based genomic DNA profiling technologies in clinical molecular diagnostics. Clin Chem. 2009;55:659-669. 66. Voelkerding KV, Dames SA, Durtschi JD. Next-generation sequencing: from basic research to diagnostics. Clin Chem. 2009;55:641-658. 67. Mullis KB, Faloona FA. Specific synthesis of DNA in vitro via a polymerase-catalyzed chain reaction. Methods Enzymol. 1987;155: 335-350. 68. Southern EM. Detection of specific sequences among DNA fragments separated by gel electrophoresis. J Mol Biol. 1975;98:503-517. 69. Carey RN, Garber CC. Evaluation of methods. In: Kaplan LA, Pesce AJ, eds. Clinical Chemistry: Theory, Practice and Correlation. 2nd ed. St. Louis, MO: Mosby; 1989:290-310. 70. Clinical Laboratory Standards Institute, National Committee for Clinical Laboratory Standards. Preliminary Evaluation of Quantitative Clinical Laboratory Methods: Approved Guideline, EP10-A3. Wayne, PA: CLSI/ NCCLS; 2006. 71. Centers for Disease Control and Prevention. Current CLIA Regulations (including all changes through 01/24/2004). Available at http:// wwwn.cdc.gov/clia/regs/toc.aspx (accessed August 2010). 72. Rivers PA, Dobalian A, Germinario FA. A review and analysis of the clinical laboratory improvement amendment of 1988: compliance plans and enforcement policy. Health Care Manage Rev. 2005;30: 93-102. 73. Jennings L, Van Deerlin VM, Gulley ML. Recommended principles and practices for validating clinical molecular pathology tests. Arch Pathol Lab Med. 2009;133:743-755. 74. Clinical Laboratory Standards Institute, National Committee for Clinical Laboratory Standards. Method Comparison and Bias Estimation Using Patient Samples: Approved Guideline, EP09-A2. Wayne, PA: CLSI/NCCLS; 2002. 75. Hilleman MR. International biological standardization in historic and contemporary perspective. Dev Biol Stand. 1999;100:19-30. 76. Rose MP. Follicle stimulating hormone international standards and reference preparations for the calibration of immunoassays and bioassays. Clin Chim Acta. 1998;273:103-117. 77. Taylor BN, Thompson A, eds. The International System of Units (SI). NIST Special Publication 330. Washington, DC: National Institute of Standards and Technology, U.S. Department of Commerce; 2008. 78. Linnet K. Necessary sample size for method comparison studies based on regression analysis. Clin Chem. 1999;45:882-894. 79. The European Parliament and the Council of the European Union Directive 98/79/EC of the European Parliament and of the Council of October 27, 1998, on in vitro diagnostic medical devices. OJ L 220, 30.8. 1993:23. 80. Powers DM. Regulations and standards: traceability of assay calibrators—the EU’s IVD directive raises the bar. IVD Technol. 2000:26-33. 81. Fraser CG, Petersen PH. Analytical performance characteristics should be judged against objective quality specifications. Clin Chem. 1999; 45:321-323. 82. Clinical Laboratory Standards Institute, National Committee for Clinical Laboratory Standards. Evaluation of Precision Performance of Quantitative Measurement Methods: Approved Guideline, EP05-A2. Wayne, PA: CLSI/NCCLS; 2004. 83. Fraser CG. Biological variation in clinical chemistry: an update— collated data, 1988-1991. Arch Pathol Lab Med. 1992;116:916-923. 84. Fraser CG, Petersen PH, Ricos C, et al. Proposed quality specifications for the imprecision and inaccuracy of analytical systems for clinical chemistry. Eur J Clin Chem Clin Biochem. 1992;30:311-317. 85. Stockl D, Baadenhuijsen H, Fraser CG, et al. Desirable routine analytical goals for quantities assayed in serum. Discussion paper from the members of the External Quality Assessment (EQA) Working Group A on analytical goals in laboratory medicine. Eur J Clin Chem Clin Biochem. 1995;33:157-169. 86. Kroll MH, Emancipator K. A theoretical evaluation of linearity. Clin Chem. 1993;39:405-413. 87. Clinical Laboratory Standards Institute, National Committee for Clinical Laboratory Standards. Evaluation of Matrix Effects: Proposed Guideline, EP14-P. Wayne, PA: CLSI/NCCLS; 1998. 88. Clinical Laboratory Standards Institute, National Committee for Clinical Laboratory Standards. Evaluation of the Linearity of Quantitative Measurement Procedures—A Statistical Approach: Approved Guideline, EP06-A. Wayne, PA: CLSI/NCCLS; 2003. 89. Clinical Laboratory Standards Institute, National Committee for Clinical Laboratory Standards. Interference Testing in Clinical Chemistry: Approved Guideline, EP07-A2. Wayne, PA: CLSI/NCCLS; 2005. 90. Levine S, Noth R, Loo A, et al. Anomalous serum thyroxin measurements with the Abbott TDx procedure. Clin Chem. 1990;36: 1838-1840.

91. Mbuyi-Kalala A, Ehrenstein G. Anomalous effects of hormone fragments on the measurement of parathyroid hormone by radioimmunoassay. Methods Find Exp Clin Pharmacol. 1996;18:87-99. 92. Micallef JV, Hayes MM, Latif A, et al. Serum binding of steroid tracers and its possible effects on direct steroid immunoassay. Ann Clin Biochem. 1995;32(Pt 6):566-574. 93. Cook NJ, Read GF. Oestradiol measurement in women on oral hormone replacement therapy: the validity of commercial test kits. Br J Biomed Sci. 1995;52:97-101. 94. Thomas CM, van den Berg RJ, Segers MF, et al. Inaccurate measurement of 17 beta-estradiol in serum of female volunteers after oral administration of milligram amounts of micronized 17 beta-estradiol. Clin Chem. 1993;39:2341-2342. 95. O’Brien PC, Dyck PJ. Procedures for setting normal values. Neurology. 1995;45:17-23. 96. Solberg HE. Establishment and use of reference values. In: Burtis CA, Ashwood E, Bruns DE, eds. Tietz Textbook of Clinical Chemistry and Molecular Diagnostics. 2nd ed. Philadelphia, PA: Saunders; 1994: 454-484. 97. Clinical Laboratory Standards Institute, National Committee for Clinical Laboratory Standards. How to Define and Determine Reference Intervals in the Clinical Laboratory: Approved Guideline, C28-A2. Wayne, PA: CLSI/NCCLS; 2000. 98. Klee GG. Maximizing efficacy of endocrine tests: importance of decision-focused testing strategies and appropriate patient preparation. Clin Chem. 1999;45:1323-1330. 99. Heins M, Heil W, Withold W. Storage of serum or whole blood samples? Effects of time and temperature on 22 serum analytes. Eur J Clin Chem Clin Biochem. 1995;33:231-238. 100. Tateishi K, Klee GG, Cunningham JM, Lennon VA. Stability of bombesin in serum, plasma, urine, and culture media. Clin Chem. 1985;31: 276-278. 101. Demir A, Alfthan H, Stenman UH, Voutilainen R. A clinically useful method for detecting gonadotropins in children: assessment of luteinizing hormone and follicle-stimulating hormone from urine as an alternative to serum by ultrasensitive time-resolved immunofluorometric assays. Pediatr Res. 1994;36:221-226. 102. Hourd P, Edwards R. Current methods for the measurement of growth hormone in urine. Clin Endocrinol (Oxf). 1994;40:155-170. 103. Leppaluoto J, Ruskoaho H. Atrial natriuretic peptide, renin activity, aldosterone, urine volume and electrolytes during a 24-h sleep-wake cycle in man. Acta Physiol Scand. 1990;139:47-53. 104. Maes M, Mommen K, Hendrickx D, et al. Components of biological variation, including seasonality, in blood concentrations of TSH, TT3, FT4, PRL, cortisol and testosterone in healthy volunteers. Clin Endocrinol (Oxf). 1997;46:587-598. 105. Sebastian-Gambaro MA, Liron-Hernandez FJ, Fuentes-Arderiu X. Intraand inter-individual biological variability data bank. Eur J Clin Chem Clin Biochem. 1997;35:845-852. 106. Demers LM. Pituitary function. In: Burtis CA, Ashwood R, eds. Tietz Textbook of Clinical Chemistry. 3rd ed. Philadelphia, PA: WB Saunders; 1999:1470-1475. 107. Abe T, Ludecke DK. Recent primary transnasal surgical outcomes associated with intraoperative growth hormone measurement in acromegaly. Clin Endocrinol (Oxf). 1999;50:27-35. 108. Bergenfelz A, Isaksson A, Lindblom P, et al. Measurement of parathyroid hormone in patients with primary hyperparathyroidism undergoing first and reoperative surgery. Br J Surg. 1998;85:11291132. 109. Baid SK, Sinaii N, Wade M, et al. Radioimmunoassay and tandem mass spectrometry measurement of bedtime salivary cortisol levels: a comparison of assays to establish hypercortisolism. J Clin Endocrinol Metab. 2007;92:3102-3107. 110. Chiu SK, Collier CP, Clark AF, et al. Salivary cortisol on ROCHE Elecsys immunoassay system: pilot biological variation studies. Clin Biochem. 2003;36:211-214. 111. Choo RE, Huestis MA. Oral fluid as a diagnostic tool. Clin Chem Lab Med. 2004;42:1273-1287. 112. Hansen AM, Garde AH, Christensen JM, et al. Evaluation of a radioimmunoassay and establishment of a reference interval for salivary cortisol in healthy subjects in Denmark. Scand J Clin Lab Invest. 2003;63:303-310. 113. Viardot A, Huber P, Puder JJ, et al. Reproducibility of nighttime salivary cortisol and its use in the diagnosis of hypercortisolism compared with urinary free cortisol and overnight dexamethasone suppression test. J Clin Endocrinol Metab. 2005;90:5730-5736. 114. Granger DA, Schwartz EB, Booth A, et al. Assessing dehydroepiandrosterone in saliva: a simple radioimmunoassay for use in studies of children, adolescents and adults. Psychoneuroendocrinology. 1999; 24:567-579. 115. O’Rorke A, Kane MM, Gosling JP, et al. Development and validation of a monoclonal antibody enzyme immunoassay for measuring progesterone in saliva. Clin Chem. 1994;40:454-458. 116. Vining RF, McGinley RA. The measurement of hormones in saliva: possibilities and pitfalls. J Steroid Biochem. 1987;27:81-94.

Laboratory Techniques for Recognition of Endocrine Disorders   99 117. Howe CJ, Handelsman DJ. Use of filter paper for sample collection and transport in steroid pharmacology. Clin Chem. 1997;43:14081415. 118. Worthman CM, Stallings JF. Hormone measures in finger-prick blood spot samples: new field methods for reproductive endocrinology. Am J Phys Anthropol. 1997;104:1-21. 119. Kricka LJ. Miniaturization of analytical systems. Clin Chem. 1998; 44:2008-2014. 120. Gabriely I, Wozniak R, Mevorach M, et al. Transcutaneous glucose measurement using near-infrared spectroscopy during hypoglycemia. Diabetes Care. 1999;22:2026-2032. 121. Tamada JA, Garg S, Jovanovic L, et al. Noninvasive glucose monitoring: comprehensive clinical results. Cygnus Research Team. JAMA. 1999;282:1839-1844. 122. Garg S, Zisser H, Schwartz S, et al. Improvement in glycemic excursions with a transcutaneous, real-time continuous glucose sensor: a randomized controlled trial. Diabetes Care. 2006;29:44-50. 123. Browning MC. Analytical goals for quantities used to assess thyrometabolic status. Ann Clin Biochem. 1989;26(Pt 1):1-12. 124. Westgard JO, Klee GG. Quality management. In: Burtis CA, Ashwood E, Bruns DE, eds. Tietz Textbook of Clinical Chemistry and Molecular Diagnostics. 4th ed. Philadelphia, PA: WB Saunders; 2006:485-531.

125. Tietz NW. Accuracy in clinical chemistry—does anybody care? Clin Chem. 1994;40:859-861. 126. Westgard JO, Barry PL, Hunt MR, et al. A multi-rule Shewhart chart for quality control in clinical chemistry. Clin Chem. 1981;27: 493-501. 127. Klee GG, Schryver PG, Kisabeth RM. Analytic bias specifications based on the analysis of effects on performance of medical guidelines. Scand J Clin Lab Invest. 1999;59:509-512. 128. Smith FA, Kroft SH. Optimal procedures for detecting analytic bias using patient samples. Am J Clin Pathol. 1997;108:254-268. 129. Nicholson S, Fox M, Epenetos A, et al. Immunoglobulin inhibiting reagent: evaluation of a new method for eliminating spurious elevations in CA125 caused by HAMA. Int J Biol Markers. 1996;11: 46-49. 130. Reinsberg J. Different efficacy of various blocking reagents to eliminate interferences by human antimouse antibodies with a two-site immunoassay. Clin Biochem. 1996;29:145-148. 131. Wheeler MJ, D’Souza A, Matadeen J, et al. Ciba Corning ACS:180 testosterone assay evaluated. Clin Chem. 1996;42:1445-1459.

Historical Perspective,  103 Neural Control of Endocrine Secretion,  104 Hypothalamic-Pituitary Unit,  106 Circumventricular Organs,  109 Pineal Gland,  113 Hypophyseotropic Hormones and Neuroendocrine Axes,  115 Neuroendocrine Disease,  157

CHAPTER CHAPTER 7  Neuroendocrinology MALCOLM J. LOW

HISTORICAL PERSPECTIVE The field of neuroendocrinology has expanded from its original focus on the control of pituitary hormone secretion by the hypothalamus to encompass multiple reciprocal interactions between the central nervous system (CNS) and endocrine systems in the control of homeostasis and physiologic responses to environmental stimuli. Although many of these concepts are relatively recent, the intimate interaction of the hypothalamus and the pituitary gland was recognized more than a century ago. For example, at the end of the 19th century, clinicians including Alfred Fröhlich described an obesity and infertility condition referred to as adiposogenital dystrophy in patients with sellar tumors.1 This condition subsequently became known as Fröhlich’s syndrome and was most often associated with the accumulation of excessive subcutaneous fat, hypogonadotropic hypogonadism (HH), and growth retardation. Whether this syndrome was a result of injury to the pituitary gland itself or to the overlying hypothalamus was extremely controversial. Several leaders in the field of endocrinology, including Cushing and his colleagues, argued that the syndrome was caused by disruption of the pituitary gland.2 However, experimental evidence began to accumulate that the hypothalamus was somehow involved

in the control of the pituitary gland. For example, Aschner demonstrated in dogs that precise removal of the pituitary gland without damage to the overlying hypothalamus did not result in obesity.3 Later, seminal studies by Hetherington and Ranson demonstrated that stereotaxic destruction of the medial basal hypothalamus with electrolytic lesions, sparing the pituitary gland, resulted in morbid obesity and neuroendocrine derangements similar to those of the patients described by Fröhlich.4 This and subsequent studies clearly established that an intact hypothalamus is required for normal endocrine function. However, the mechanisms by which the hypothalamus was involved in endocrine regulation remained unsettled for years. It is now thought that the phenotypes of Fröhlich’s syndrome and of the ventromedial hypothalamic lesion syndrome are caused by dysfunction or destruction of key hypothalamic neurons that regulate pituitary hormone secretion and energy homeostasis. The field of neuroendocrinology took a major step forward when several groups, especially that of Ernst and Berta Scharrer, recognized that neurons in the hypothalamus are the source of the axons that constitute the neural lobe (see “Neurosecretion”). The hypothalamic control of the anterior pituitary gland remained unclear, however. Popa and Fielding identified the pituitary portal vessels linking the median eminence of the hypothalamus and the 103

104    Neuroendocrinology anterior pituitary gland.5 Although they appreciated the fact that this vasculature provided a link between the hypothalamus and the pituitary gland, they initially hypothesized that blood flowed from the pituitary up to the brain. Anatomic studies by Wislocki and King supported the concept that blood flow was from the hypothalamus to the pituitary.6 Later studies, including the seminal work of Geoffrey Harris, established the flow of blood from the hypothalamus at the median eminence to the anterior pituitary gland.7 This supported the concept that the hypothalamus controls anterior pituitary gland function indirectly and led to the now accepted hypophyseal-portal chemotransmitter hypothesis. Subsequently, several important studies, especially those from Schally and colleagues and from the Guillemin group, established that the anterior pituitary is tightly controlled by the hypothalamus.8,9 Both groups identified several putative peptide hormone-releasing factors (see later sections). These fundamental studies resulted in the awarding of the Nobel Prize in Medicine in 1977 to Andrew Schally and Roger Guillemin.10,11 We now know that these releasing factors are the fundamental link between the CNS and the control of endocrine function. Furthermore, these neuropeptides are highly conserved across species and are essential for reproduction, growth, and metabolism. The anatomy, physiology, and genetics of these releasing factors constitute a major portion of this chapter. Over the past 2 decades, work in the field of neuroendocrinology has continued to advance across several fronts. Cloning and characterization of the specific G protein– coupled receptors used by the hypothalamic releasing factors have helped define their signaling mechanisms. Characterization of the distribution of these receptors has universally demonstrated receptor expression in the brain and in peripheral tissues other than the pituitary, arguing for multiple physiologic roles for the neuropeptidereleasing factors. Finally, the last 2 decades have also seen tremendous advances in understanding of regulatory neuronal and humoral inputs to the hypophyseotropic neurons. The adipostatic hormone leptin, discovered in 1994,12 is an example of a humoral factor that has profound effects on multiple neuroendocrine circuits. Reduction in circulating leptin is responsible for suppression of the thyroid and reproductive axes during the starvation response. The subsequent discovery of ghrelin,13 a stomach peptide that regulates appetite and also acts on multiple neuroendocrine axes, demonstrated that much remains to be learned regarding the regulation of the hypothalamic releasing hormones. Traditionally, it has been extremely difficult to study releasing-factor gene expression or the specific regulation of the releasing factor neurons because of their small numbers and, in some cases, diffuse distribution. Transgenic experiments have produced mice in which expression of fluorescent marker proteins is specifically targeted to gonadotropin-releasing hormone (GnRH) neurons14 and arcuate pro-opiomelanocortin (POMC) neurons,15 among others. This technology allows detailed study of the electrophysiologic properties of hypothalamic neurons in the more native context of slice preparations or organotypic cultures. Although much of the field of neuroendocrinology has focused on hypothalamic releasing factors and their influence on reproduction, growth, development, fluid balance, and the stress response through control of pituitary hormone production, the term neuroendocrinology has come to mean the study of the interactions of the endocrine and nervous systems in the regulation of homeostasis. The field of neuroendocrinology has been further

expanded by input from diverse areas of basic research that has often been fundamental to understanding of the neuroendocrine system. These areas include studies of neuropeptide structure, function, and mechanism of action; neural secretion; hypothalamic neuroanatomy; G protein– coupled receptor structure, function, and signaling; transport of substances into the brain; and the action of hormones on the brain. Moreover, homeostatic systems often involve integrated endocrine, autonomic, and behavioral responses. In many of these systems (e.g., energy homeostasis, immune function), the classic neuroendocrine axes are important but not autonomous pathways, and these subjects are also often studied in the context of neuroendocrinology. In this chapter, the concepts of neural secretion, the neuroanatomy of the hypothalamic-pituitary unit, and the CNS structures most relevant to control of the neurohypophysis and adenohypophysis are presented. Then, each classic hypothalamic-pituitary axis is described, including a consideration of the immune system and its integration with neuroendocrine function. Finally, the pathophysiology of disorders of neural regulation of endocrine function are reviewed. The neuroendocrinology of energy homeostasis is fully considered in Chapter 35.

NEURAL CONTROL OF ENDOCRINE SECRETION A fundamental principle of neuroendocrinology encompasses the regulated secretion of hormones, neurotransmitters, or neuromodulators by specialized cells.16 Endocrine cells and neurons are prototypical secretory cells. Both have electrically excitable plasma membranes and specific ion conductances that regulate exocytosis of their signaling molecules from storage vesicles. Secretory cells are broadly classified by their topographic mechanisms of secretion. For example, endocrine cells secrete their contents directly into the bloodstream, allowing these substances to act globally as hormones. Cells classified as paracrine secrete their contents into the extracellular space and predominantly affect the function of closely neighboring cells. Autocrine secretory cells affect their own function by the local actions of their secretions. In contrast, secretory cells within exocrine glands secrete proteinaceous substances, including enzymes, and lipids into the lumen of ductal systems.

Neurosecretion Neurons are secretory cells that send their axons throughout the nervous system to release their neurotransmitters and neuromodulators predominantly at specialized chemical synapses. Neurohumoral or neurosecretory cells constitute a unique subset of neurons whose axon terminals are not associated with synapses. Two examples of neurosecretory cells are neurohypophyseal and hypophyseotropic cells. The prototypical neurohypophyseal cells are the magnicellular neurons of the paraventricular (PVH) and supraoptic (SON) nuclei in the hypothalamus. Hypophyseotropic cells are neurons that secrete their products into the pituitary portal vessels at the median eminence (Fig. 7-1). In the most basic sense, neurosecretory cells are neurons that secrete substances directly into the bloodstream to act as hormones. The theory of neurosecretion evolved from the seminal work of Scharrer and Scharrer,16,17 who used morphologic techniques to identify stained secretory

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Magnicellular Neuron Location: SON, PVH (AVP, OXY)

Parvicellular Hypophyseotropic Neuron Location: PeVH, PVH (TRH, CRH, Somatostatin) Arc (GHRH, GnRH, Dopamine)

Hypothalamic Projection Neuron Location: PVH (AVP, OXY) LHA (MCH, ORX) Arc (POMC, AgRP)

Hypothalamus Neural lobe

Releasing factors

Vasopressin and oxytocin

Neuronal targets (e.g., sympathetic preganglionic neuron in spinal cord) Trophic hormones (ACTH, TSH, GH, LH, FSH, Prolactin)

Kidney, Uterus, Mammary Gland

Anterior Pituitary Gland

Neuronal Targets

Figure 7-1 Three types of hypothalamic neurosecretory cells. Left, A magnicellular neuron that secretes arginine vasopressin (AVP) or oxytocin (OXY). The cell body, which is located in the supraoptic (SON) or paraventricular hypothalamic (PVH) nucleus, projects its neuronal process into the neural lobe, and neurohormone is released from nerve endings. Center, Similar peptidergic neurons are located in the medial basal hypothalamus in nuclear groups including the periventricular hypothalamic nucleus (PeVH), the PVH, and the infundibular or arcuate nucleus of the hypothalamus (Arc). The neuropeptides in this case are released into the specialized blood supply to the pituitary to regulate its secretion. Right, A third category of hypothalamic peptidergic neurons terminates at chemical synapses on other neurons. These projection neurons are found in sites including the PVH, Arc, and lateral hypothalamic area (LHA) that innervate multiple central nervous system nuclei, including autonomic preganglionic neurons in the brain stem and spinal cord. Such substances act as neurotransmitters or neuromodulators. ACTH, corticotropin; AgRP, agouti-related peptide; CRH, corticotropin-releasing hormone; FSH, follicle-stimulating hormone; GH, growth hormone; GHRH, growth hormone–releasing hormone; GnRH, gonadotropin-releasing hormone; LH, luteinizing hormone; MCH, melanin-concentrating hormone; ORX, orexin/hypocretin; POMC, pro-opiomelanocortin; TRH, thyrotropin-releasing hormone; TSH, thyrotropin.

granules in the SON and PVH neurons. They found that cutting the pituitary stalk led to an accumulation of these granules in the hypothalamus, which led them to hypothesize that hypothalamic neurons were the source of substances secreted by the neural lobe (posterior pituitary). Although this concept initially raised great skepticism among contemporary researchers, it is now known that the axon terminals in the neural lobe arise from the SON and PVH magnicellular neurons that contain oxytocin and the antidiuretic hormone, arginine vasopressin (AVP). The modern definition of neurosecretion has evolved to include the release of any secretory product from a neuron. Indeed, a fundamental tenet of neuroscience is that all neurons in the CNS, including neurons that secrete AVP and oxytocin in the neural lobe, receive multiple synaptic inputs largely onto their dendrites and cell bodies. In addition, neurons have the basic ability to detect and integrate input from multiple neurons through specific receptors. They in turn fire action potentials that result in the release of neurotransmitters and neuromodulators into synapses formed with postsynaptic neurons. Most communications between neurons are accomplished by the classic fastacting neurotransmitters (e.g., glutamate, γ-aminobutyric acid [GABA], acetylcholine) and neuromodulators (e.g.,

neuropeptides) acting at chemical synapses.11,18 Neurosecretion represents a fundamental concept in understanding the mechanisms used by the nervous system to control behavior and maintain homeostasis. In the era of the elucidation of the human genome, the importance of these early observations is often not fully appreciated. However, accounts of these early studies are illuminating, and it is not an overstatement that confirmation of the neurosecretion hypothesis represented one of the major advances in the fields of neuroscience and neuroendocrinology. Indeed, these and other early experiments, including the pioneering work of Geoffrey Harris,7,19 led to the fundamental concept that the hypothalamus releases hormones directly into the bloodstream (neurohypophyseal cells). These observations provided the principles on which the modern discipline of neuroendocrinology is built.

Contribution of the Autonomic Nervous System to Endocrine Control Another major precept of neuroendocrinology is that the nervous system controls or modifies the function of both endocrine and exocrine glands. The exquisite control of

106    Neuroendocrinology the anterior pituitary gland is accomplished by the action of releasing factor hormones (see “Hypophyseotropic Hormones and Neuroendocrine Axes”). Other endocrine and exocrine organs (e.g., pancreas, adrenal, pineal, salivary glands) are also regulated through direct innervation from the cholinergic and noradrenergic inputs from the autonomic nervous system. Although it is beyond the scope of this chapter, an appreciation of the functional anatomy and pharmacology of the parasympathetic and sympathetic nervous systems is fundamental to understanding of the neural control of endocrine function. The efferent arms of the autonomic nervous system comprise the sympathetic and parasympathetic systems. These have similar wiring diagrams characterized by a preganglionic neuron that innervates a postganglionic neuron that in turn targets an end organ.20 Preganglionic and postganglionic parasympathetic neurons are cholinergic. In contrast, preganglionic sympathetic neurons are cholinergic and postganglionic neurons are noradrenergic (except for those innervating sweat glands, which are cholinergic). Another basic concept is that autonomic neurons coexpress several neuropeptides. This coexpression is a common feature of neurons in the central and peripheral nervous systems.11,18,21 For example, postganglionic noradrenergic neurons coexpress somatostatin and neuropeptide Y (NPY). Postganglionic cholinergic neurons coexpress neuropeptides including vasoactive intestinal polypeptide (VIP) and calcitonin gene–related peptide (CGRP). Most sympathetic preganglionic neurons lie in the intermediolateral cell column in the thoracolumbar regions of the spinal cord.20 Most postganglionic sympathetic neurons are located in sympathetic ganglia lying near the vertebral column (e.g., sympathetic chain, superior cervical ganglia). Postganglionic fibers innervate target organs. As a rule, the sympathetic preganglionic fibers are relatively short, and the postganglionic fibers are long. In contrast, the parasympathetic preganglionic neurons lie in the midbrain (perioculomotor area, long misidentified as the Edinger-Westphal nucleus22), the medulla oblongata (e.g., dorsal motor nucleus of the vagus, nucleus ambiguus), and the sacral spinal cord. Postganglionic neurons that innervate the eye and salivary glands arise from the ciliary, pterygopalatine, submandibular, and otic ganglia. Postganglionic parasympathetic neurons in the thorax and abdomen typically lie within the target organs, including the gut wall and pancreas.20 Consequently, the parasympathetic preganglionic fibers are relatively long, and the postganglionic fibers are short. A dual autonomic innervation of the pancreas illustrates the importance of coordinated neural control of endocrine organs. The endocrine pancreas receives both sympathetic (noradrenergic) and parasympathetic (cholinergic) innervation.20,23 The latter activity is provided by the vagus nerve (dorsal motor nucleus of the vagus) and is an excellent example of neural modulation, because the cholinergic tone of the beta cells affects their secretion of insulin. For example, vagal input is thought to modulate insulin secretion before (cephalic phase), during, and after ingestion of food.24 In addition, noradrenergic stimulation of the endocrine pancreas can alter the secretion of glucagon and inhibits insulin release.23 Of course, a major regulator of insulin secretion is the extracellular concentration of glucose,25 and glucose can induce insulin secretion in the absence of neural input. However, the exquisite control by the nervous system is illustrated by the fact that populations of neurons in the brain stem and hypothalamus have the ability, like the beta cell, to sense glucose levels in the bloodstream.26 This information is integrated by the

hypothalamus and ultimately results in alterations in the activity of the autonomic nervous system innervating the pancreas. Therefore, neural control of the endocrine pancreas probably contributes to the physiologic control of insulin secretion and may contribute to the pathophysiology of disorders such as diabetes mellitus. Certainly, an increased understanding of this complex interplay between the CNS and endocrine function is necessary to diagnose and clinically manage endocrine disorders.

HYPOTHALAMIC-PITUITARY UNIT The hypothalamus is one of the most evolutionarily conserved and essential regions of the mammalian brain. Indeed, the hypothalamus is the ultimate brain structure that allows mammals to maintain homeostasis, and destruction of the hypothalamus is not compatible with life. Hypothalamic control of homeostasis stems from the ability of this collection of neurons to orchestrate coordinated endocrine, autonomic, and behavioral responses. A key principle is that the hypothalamus receives sensory inputs from the external environment (e.g., light, nociception, temperature, odorants) and information regarding the internal environment (e.g., blood pressure, blood osmolality, blood glucose levels). Of particular relevance to neuroendocrine control, hormones (e.g., glucocorticoids, estrogen, testosterone, thyroid hormone) exert both negative and positive feedback directly on the hypothalamus. The hypothalamus integrates diverse sensory and hormonal inputs and provides coordinated responses through motor outputs to key regulatory sites. These include the anterior pituitary gland, posterior pituitary gland, cerebral cortex, premotor and motor neurons in the brain stem and spinal cord, and parasympathetic and sympathetic preganglionic neurons. The patterned hypothalamic outputs to these effector sites ultimately result in coordinated endocrine, behavioral, and autonomic responses that maintain homeostasis. The hypothalamic control of the pituitary gland is an elegant system that underlies the ability of mammals to coordinate endocrine functions that are necessary for survival.

Development and Differentiation of Hypothalamic Nuclei Tremendous advances in knowledge of the molecular and genetic basis for embryonic development of the hypothalamic-pituitary unit have occurred in the past 2 decades as a result of the genome sequencing projects and use of transgenic model systems.27 Pituitary development is discussed in detail in Chapter 8, and only a few key points most relevant to the physiology and pathophysiology of the neuroendocrine hypothalamus are presented here. There has been considerable debate concerning the extent to which developmental studies in the rodent hypothalamic-pituitary system are applicable to the human. However, accumulating data suggest that the similarities outweigh the differences. Ontogenic analyses of the organization of the human hypothalamus using a battery of neurochemical markers have reinforced its homologies to the better-studied rat brain.28 The cytoarchitectonic boundaries of hypothalamic nuclei are much more easily discerned in fetal human brain than in the adult brain and for the most part correspond to homologous structures in the rat hypothalamus. This finding has important implications for the validity of interspecies comparative analyses.

Neuroendocrinology    107

Two examples further illustrate this point. First, the ventromedial nucleus of the hypothalamic core (ventromedial hypothalamus, or VMH), which plays a role in energy balance and in female sexual behavior, differentiates from neuroblasts in both humans and rodents at a time point that is intermediate between the earlier differentiation of lateral hypothalamic nuclei and later differentiation of the midline nuclei, including the suprachiasmatic nucleus (SCN), the arcuate nucleus, and the PVH.28,29 Expression of the transcription factor SF1 has been shown to be restricted both temporally and spatially to cells in the VMH, and knockout of the SF1 gene in mice alters VMH development by influencing the migration of cells and their ultimate location.29 A second example of interspecies homologies in hypothalamic development is the migration of GnRHsecreting neurons from their origins in rostral neuroepithelium to the anterior hypothalamus.30 As discussed later, spontaneous and inherited mutations in genes that affect the migration of these neurons are an important cause of Kallmann’s syndrome or HH associated with anosmia. In addition to SF1 and the genes associated with Kallmann’s syndrome, a growing list of genes primarily encoding transcription factors have been implicated in human neuroendocrine disorders and characterized experimentally in rodent models.31 This list includes the homeobox transcription factor OTP and the heterodimeric complex formed by the basic helix-loop-helix (bHLH) factors SIM1 and ARNT2. These factors are required for the proper development of the PVH and the SON and for expression of many key hypophyseotropic neuropeptide genes. The physiologic importance of SIM1 is illustrated by the development of an obesity phenotype in both mice and humans with a haploinsufficiency of SIM1 expression.31 MASH1 and its downstream target GSH1 are both critical for neuron specification and expression of growth hormone–releasing hormone (GHRH).32 Two key concepts involved in CNS development, which also apply to the hypothalamus, are the balance between neurogenesis and cell death in the establishment of nuclei and the role of circulating hormones in providing organizational signals that regulate cell number and synaptic remodeling. The most thoroughly characterized examples are the effects of sex steroid hormones on the developing brain, which result in key sexual dimorphisms of functional importance in later reproductive behaviors.33 This principle has been extended to include organizational effects of other classes of hormones. For example, leptin plays an important role in the development of medial-basal hypothalamic circuits that are important for energy homeostasis by mediating axonal projections between hypothalamic nuclei.34

Anatomy of the Hypothalamic- Pituitary Unit The pituitary gland is regulated by three interacting elements: hypothalamic inputs (releasing factors or hypo­ physeotropic hormones), feedback effects of circulating hormones, and paracrine and autocrine secretions of the pituitary itself. In humans, the pituitary gland (hypo­ physis) can be divided into two major parts, the adeno­ hypophysis and the neurohypophysis, which are easily distinguishable on T1-weighted magnetic resonance imaging (MRI) (Fig. 7-2).35 The adenohypophysis can be subdivided into three distinct lobes: the pars distalis (anterior lobe), pars intermedia (intermediate lobe), and pars tuberalis. Whereas a well-developed intermediate lobe is found in most mammals, only rudimentary vestiges of the

intermediate lobe are detectable in adult humans, with the bulk of intermediate lobe cells being dispersed in the anterior and posterior lobes. The neurohypophysis is composed of the pars nervosa (also known as the neural or posterior lobe), the infundibular stalk, and the median eminence. The infundibular stalk is surrounded by the pars tuberalis, and together they constitute the hypophyseal stalk. The pituitary gland lies in the sella turcica (“Turkish saddle”) of the sphenoid bone and underlies the base of the hypothalamus. This anatomic location explains the hypothalamic damage described by Fröhlich.1 In humans, the base of the hypothalamus forms a mound called the tuber cinereum, the central region of which gives rise to the median eminence (see Fig. 7-2).36 The anterior and intermediate lobes of the pituitary derive from a dorsal invagination of the pharyngeal epithelium, called Rathke’s pouch, in response to inductive signals from the overlying neuroepithelium of the ventral diencephalon. During development, precursor cells within the pouch undergo steps of organ determination, cell fate commitment to a pituitary phenotype, proliferation, and migration.27 The intermediate lobe is in direct contact with the neural lobe and is the least prominent of the three lobes. With age, the human intermediate lobe decreases in size to leave a small, residual collection of POMC cells. In nonprimate species, these cells are responsible for secreting the POMC-derived product α-melanocyte–stimulating hormone (α-MSH).37 The major component of the neural lobe is a collection of axon terminals arising from magnicellular secretory neurons located in the PVH and SON nuclei of the hypothalamus (Fig. 7-3; see Fig. 7-1). These axon terminals are in close association with a capillary plexus, and they secrete substances including AVP and oxytocin into the hypophyseal veins and thence into the general circulation (Table 7-1). The blood supply to the neurohypophysis arises from the inferior hypophyseal artery (a branch of the internal carotid artery). Glial-like cells called pituicytes are scattered among the nerve terminals. As the source of AVP to the general circulation, the PVH and SON nuclei and their axon terminals in the neural lobe are the effector arms for the central regulation of blood osmolality, fluid balance, and blood pressure (see Chapter 10). The secretion of oxytocin by magnicellular neurons is critical at parturition, resulting in uterine myometrial contraction. In addition, the secretion of oxytocin is regulated by the classic milk let-down reflex.38 Although the exact neuroanatomic substrate underlying this response is unclear, apparently mechanosensory information from the nipple reaches the magnicellular neurons, directly or indirectly, from the dorsal horn of the spinal cord, resulting in release of oxytocin into the general circulation.39 Oxytocin acts on receptors on myoepithelial cells in the mammary gland acini, leading to release of milk into the ductal system of the mammary gland.

The Median Eminence and Hypophyseotropic Neuronal System The median eminence is the functional link between the hypothalamus and the anterior pituitary gland. It lies in the center of the tuber cinereum and is composed of an extensive array of blood vessels and nerve endings (Fig. 7-4; see Fig. 7-2).17,36,40 Its extremely rich blood supply arises from the superior hypophyseal artery (a branch of the internal carotid artery), which sends off many small branches that form capillary loops. The small capillary loops extend into the internal and external zones of the

108    Neuroendocrinology

A

B Figure 7-2  Normal anatomy of the human hypothalamic-pituitary unit in sagittal (A) and coronal (B) planes. Structures that are visible in the T1-weighted magnetic resonance images (left panels) are identified in the corresponding diagrams (right panels). The hypothalamus is bounded anteriorly by the optic chiasm, laterally by the sulci formed with the temporal lobes, and posteriorly by the mammillary bodies (in which the mammillary nuclei are located). Dorsally, the hypothalamus is delineated from the thalamus by the hypothalamic sulcus. The smooth, rounded base of the hypothalamus is the tuber cinereum; the pituitary stalk descends from its central region, which is termed the median eminence. The median eminence stands out from the rest of the tuber cinereum because of its dense vascularity, which is formed by the primary plexus of the hypophyseal-portal system. The long portal veins run along the ventral surface of the pituitary stalk. Notice the location of the pituitary stalk, the hyperintense signal (white) from the posterior pituitary (PP) (panel A, left), and the anatomic relationships of the pituitary gland to the optic chiasm (oc) and the sphenoidal and cavernous sinuses. ac, anterior commissure; AP, anterior pituitary;  cc, corpus callosum; MB, mammillary body; pc, posterior commissure. (Magnetic resonance images courtesy of Dr. D.M. Cook.)

median eminence, form anastomoses, and drain into sinusoids that become the pituitary portal veins that enter the vascular pool of the pituitary gland.40-42 The flow of blood in these short loops is thought to be predominantly (if not exclusively) in a hypothalamic-to-pituitary direction.42 This well-developed plexus results in a tremendous increase in the vascular surface area. In addition, the vessels are fenestrated, allowing diffusion of the peptide-releasing factors to their site of action in the anterior pituitary gland. Because this vascular complex in the base of the hypothalamus and its “arteriolized” venous drainage to the pituitary compose a circulatory system analogous to the portal vein system of the liver, it has been termed the hypophysealportal circulation. Three distinct compartments of the median eminence are recognized: the innermost ependymal layer, the internal zone, and the external zone (see Fig. 7-4).40 Ependymal cells form the floor of the third ventricle and are unique in that they have microvilli rather than cilia. Tight junctions at the ventricular pole of the ependymal cells prevent the diffusion of large-molecular-weight substances between

the cerebrospinal fluid (CSF) and the extracellular space within the median eminence. The ependymal layer also contains specialized cells, called tanycytes, that send processes into the other layers of the median eminence.43 Tight junctions between tanycytes at the lateral edges of the median eminence likely prevent the diffusion of releasing factors back into the medial basal hypothalamus. The internal zone of the median eminence is composed of axons of the SON and PVH magnicellular neurons passing en route to the posterior pituitary (see Fig. 7-4C) and axons of the hypophyseotropic neurons destined for the external layer of the median eminence (see Fig. 7-4A and B). In addition, supportive cells populate this layer. Finally, the external zone of the median eminence represents the exchange point of the hypothalamic releasing factors and the pituitary portal vessels.40 Two general types of tuberohypophyseal neurons project to the external zone: peptide-secreting (peptidergic) neurons including thyrotropin-releasing hormone (TRH), corticotropinreleasing hormone (CRH), and GnRH (see Fig. 7-1) and neurons containing monoamines (e.g., dopamine,

Neuroendocrinology    109

A

B

C

D

Figure 7-3 The tuberoinfundibular system is revealed by retrograde transport of cholera toxin subunit B (CtB). The location of hypothalamic cell bodies of neurons projecting to the median eminence (ME) and the posterior pituitary are identified by microinjection of a small volume of the retrograde tracer CtB into the median eminence of the rat. A, Retrogradely labeled cells can be seen in the paraventricular (PVH) and supraoptic (SON) nuclei of the hypothalamus. B, Magnicellular neurons are observed in the SON. C, Labeled neurons are found in the posterior magnicellular group (pm) and in the medial parvicellular subdivision (mp). The labeled cells in the PVH include those that contain corticotropin-releasing hormone and thyrotropin-releasing hormone. D, Retrogradely labeled cells are also found in the arcuate nucleus of the hypothalamus (Arc). These include neurons that secrete growth hormone–releasing hormone and dopamine. 3v, third ventricle; ot, optic tract. (Photomicrographs courtesy of Dr. R. M. Lechan.)

serotonin). Although the secretion of these substances into the portal circulation is an important control mechanism, some peptides and neurotransmitters in nerve endings are not released into the hypophyseal-portal circulation but instead function to regulate the secretion of other nerve terminals.44 The anatomic relationships of nerve endings, basement membranes, interstitial spaces, fenestrated (windowed) capillary endothelia, and glia in the median eminence are similar to those in the neural lobe. As in the case of neurohormone secretion from the neurohypophysis, depolarization of hypothalamic cells leads to the release of neuropeptides and monoamines at the median eminence. Non-neuronal supporting cells in the hypothalamus also play a dynamic role in hypophyseotropic regulation. For example, nerve terminals in the neurohypophysis are enveloped by pituicytes; they surround the nerve endings when the gland is inactive but retract to expose the terminals when AVP secretion is enhanced, as in states of dehydration. Within the median eminence, GnRH nerve endings are enveloped by the tanycytes, which also cover or uncover neurons with changes in functional status.43,45 Thus, supporting elements, with their own sets of receptors, can change the neuroregulatory milieu within the hypothalamus, median eminence, and pituitary. The site of production, the genetics, and the regulation of synthesis and release of individual peptide-releasing factors are discussed in detail in later sections. Briefly, there are several cell groups in the medial hypothalamus that contain releasing factors that are secreted into the pituitary

portal circulation (Table 7-2).36,46 These cell groups include the infundibular nucleus (called the arcuate nucleus in rodents) (see Fig. 7-3D), the PVH (see Fig. 7-3A and C), and a group of cells in the medial preoptic area near the organum vasculosum of the lamina terminalis (OVLT) (Fig. 7-5). As discussed earlier, magnicellular neurons in the SON and PVH send axons that predominantly traverse the median eminence to terminate in the neural lobe of the pituitary. In addition, a smaller number of magnicellular axons project directly to the external zone of the median eminence, but their functional significance is unknown. The third structure often grouped as a component of the median eminence is a subdivision of the adenohypophysis called the pars tuberalis. It is a thin sheet of glandular tissue that lies around the infundibulum and pituitary stalk. In some animals, the epithelial component makes up as much as 10% of the total glandular tissue of the anterior pituitary. The pars tuberalis contains cells that make pituitary tropic hormones including luteinizing hormone (LH) and thyrotropin (thyroid-stimulating hormone, or TSH). A definitive physiologic function of the pars tuberalis is not established, but melatonin receptors are expressed in the pars tuberalis.

CIRCUMVENTRICULAR ORGANS A guiding principle of neurophysiology and neuropharmacology is that the brain, including the hypothalamus, resides in an environment that is protected from humoral

110    Neuroendocrinology TABLE 7-1 

Neurotransmitters and Neuromodulators in the Paraventricular Nucleus and the Arcuate Nucleus of the Hypothalamus Paraventricular Nucleus

Arcuate Nucleus

Magnicellular Division Angiotensin II Cholecystokinin (CCK) Dynorphins Nitric oxide (NO) Oxytocin Vasopressin (AVP)

Acetylcholine γ-Aminobutyric acid (GABA) Agouti-related peptide (AgRP) Cocaine- and amphetamineregulated transcript (CART) Dopamine Dynorphin Endocannabinoids Enkephalins Galanin Galanin-like peptide (GALP) Glutamate Gonadotropin-releasing hormone (GnRH) Growth hormone–releasing hormone (GHRH) Kisspeptins Melanocortins (ACTH, α-MSH, β-MSH, γ-MSH) Neurokinin B (NKB) Neuromedin U Neuropeptide Y (NPY) Neurotensin Nociceptin/orphanin FQ (OFQ) Opioids (β-endorphin) peptides Pancreatic polypeptide Prolactin Pro-opiomelanocortin Pyro-glutamyl-RFamide peptide (QRFP) Somatostatin Substance P

Parvicellular Divisions γ-Aminobutyric acid (GABA) Angiotensin II Atrial natriuretic factor (ANF) Bombesin-like peptides Cholecystokinin (CCK) Corticotropin-releasing hormone (CRH) Dopamine Endocannabinoids Enkephalins Galanin Glutamate Interleukin-1 (IL-1) Neuropeptide Y (NPY) Neurotensin Nitric oxide (NO) RFamide–related peptides (RFRP) Somatostatin Thyrotropin-releasing hormone (TRH) Vasopressin (AVP) Vasoactive intestinal peptide (VIP)

signals.43,47,48 The exclusion of macromolecules is achieved by the structural vascular specializations that make up the blood-brain barrier. These specializations include tight junctions of brain vascular endothelial cells that preclude the free passage of polarized macromolecules including peptides and hormones. In addition, astrocytic foot processes and perivascular microglial cells contribute to the integrity of the blood-brain barrier.48 However, to exert homeostatic control, the brain must assess key sensory information from the bloodstream, including levels of hormones, metabolites, and potential toxins. To monitor key signals, the brain has “windows on the circulation” or circumventricular organs (CVOs) that serve as a conduit of peripheral cues into key neuronal cell groups that maintain homeostasis.47,48 As the name implies, CVOs are specialized structures that lie on the midline of the brain along the third and fourth ventricles. These structures include the OVLT, subfornical organ (SFO), median eminence, neurohypophysis (posterior pituitary), subcommissural organ (SCO), and area postrema (see Fig. 7-5). Unlike the vasculature in the rest of the brain, the blood vessels in CVOs have fenestrated capillaries that allow relatively free passage of molecules such as proteins and peptide hormones. Thus, neurons and glial cells that reside within the CVOs have

access to these macromolecules. In addition to the distinct nature of the vessels themselves, the CVOs have an unusually rich blood supply, which allows them to act as integrators at the interface of the blood-brain barrier. Several of the CVOs have major projections to hypothalamic nuclear

A

B

C Figure 7-4 The median eminence (ME) is the functional connection between the hypothalamus and the pituitary gland. A and B, Distribution of corticotropin-releasing hormone (CRH) and thyrotropin-releasing hormone (TRH) immunoreactivity (IR) in the external (ext) layer of the ME of the rat. CRH and TRH cell bodies reside in the medial division of the paraventricular hypothalamic nucleus. C, Arginine vasopressin immunoreactivity (AVP-IR) in nerve endings in the internal (int) layer of the ME. Arc, Arcuate nucleus; 3v, third ventricle. (Photomicrographs courtesy of Dr. R.M. Lechan.)

Neuroendocrinology    111 TABLE 7-2 

Structural Formulas of Principal Human Hypothalamic Peptides Directly Related to Pituitary Secretion* Vasopressin Cys-Tyr-Phe-Gln-Asn-Cys-Pro-Arg-Gly-NH2 (MW = 1084.38)

groups that regulate homeostasis (see later discussion). Therefore, the CVOs serve as a critical link between peripheral metabolic cues, hormones, and potential toxins and cell groups within the brain that regulate coordinated endocrine, autonomic, and behavioral responses. Detailed discussion of the physiologic roles of individual CVOs is beyond the scope of this chapter, but several in-depth reviews have assessed the function of each.47-50

Oxytocin Cys-Tyr-Ile-Gln-Asn-Cys-Pro-Leu-Gly-NH2 (MW = 1007.35)

Median Eminence

Thyrotropin-Releasing Hormone

The median eminence and neurohypophysis contain the neurosecretory axons that control pituitary function. The role of the median eminence as a link between the hypothalamus and the pituitary gland is detailed in other sections of this chapter (see “Hypothalamic-Pituitary Unit” and Figs. 7-2 and 7-4). The anatomic location of the median eminence places it in a position to serve as an afferent sensory organ as well. Specifically, the median eminence is located adjacent to several neuroendocrine and autonomic regulatory nuclei at the tuberal level of the hypothalamus (see Fig. 7-3). These nuclear groups include the infundibular or arcuate, ventromedial, dorsomedial, and paraventricular nuclei.36 A role of the hypothalamic nuclei surrounding the median eminence as afferent sensory centers is supported by several observations. For example, toxins such as monosodium glutamate and gold thioglucose damage neurons in cell groups overlying the median eminence, resulting in obesity and hyperphagia. Experimental evidence suggests that the median eminence is a portal of entry for hormones such as leptin. Indeed, administration of radiolabeled peptides or hormones, such as α-MSH or leptin, led to their

pGlu-His-Pro-NH2 (MW = 362.42) Gonadotropin-Releasing Hormone pGlu-His-Trp-Ser-Tyr-Gly-Leu-Arg-Pro-Gly-NH2 (MW = 1182.39) Corticotropin-Releasing Hormone Ser-Glu-Glu-Pro-Pro-Ile-Ser-Leu-Asp-Leu-Thr-Phe-His-Leu-Leu-Arg-GluVal-Leu-Glu-Met-Ala-Arg-Ala-Glu-Gln-Leu-Ala-Gln-Gln-Ala-His-SerAsn-Arg-Lys-Leu-Met-Glu-Ile-Ile-NH2 (MW = 4758.14) Growth Hormone–Releasing Hormone GHRH(1-40) Tyr-Ala-Asp-Ala-Ile-Phe-Thr-Asn-Ser-Tyr-Arg-Lys-Val-Leu-Gly-Gln-Leu-SerAla-Arg-Lys-Leu-Leu-Gln-Asp-Ile-Met-Ser-Arg-Gln-Gln-Gly-Glu-SerAsn-Gln-Glu-Arg-Gly-Ala (MW = 4544.73) GHRH(1-44)-NH2 Tyr-Ala-Asp-Ala-Ile-Phe-Thr-Asn-Ser-Tyr-Arg-Lys-Val-Leu-Gly-Gln-Leu-SerAla-Arg-Lys-Leu-Leu-Gln-Asp-Ile-Met-Ser-Arg-Gln-Gln-Gly-Glu-SerAsn-Gln-Glu-Arg-Gly-Ala-Arg-Ala-Arg-Leu-NH2 (MW = 5040.4) Somatostatin Ala-Gly-Cys-Lys-Asn-Phe-Phe-Trp-Lys-Thr-Phe-Thr-Ser-Cys (MW = 1638.12) Somatostatin-28 SST-28 Ser-Ala-Asn-Ser-Asn-Pro-Ala-Met-Ala-Pro-Arg-Glu-Arg-Lys-Ala-Gly-CysLys-Asn-Phe-Phe-Trp-Lys-Thr-Phe-Thr-Ser-Cys (MW = 3149.0)

cc

SST-28(1-12) Ser-Ala-Asn-Ser-Asn-Pro-Ala-Met-Ala-Pro-Arg-Gln (MW = 1244.49)

CP

Vasoactive Intestinal Peptide His-Ser-Asp-Ala-Val-Phe-Thr-Asp-Asn-Tyr-Thr-Arg-Leu-Arg-Lys-Gln-MetAla-Val-Lys-Lys-Tyr-Leu-Asn-Ser-Ile-Leu-Asn-NH2 (MW = 3326.26) Prolactin-Releasing Peptide PrRP31 Ser-Arg-Thr-His-Arg-His-Ser-Met-Glu-Ile-Arg-Thr-Pro-Asp-Ile-Asn-ProAla-Trp-Tyr-Ala-Ser-Arg-Gly-Ile-Arg-Pro-Val-Gly-Arg-Phe-NH2 (MW = 3665.16) PrRP20 Thr-Pro-Asp-Ile-Asn-Pro-Ala-Trp-Tyr-Ala-Ser-Arg-Gly-Ile-Arg-Pro-Val-GlyArg-Phe-NH2 (MW = 2273.58) Ghrelin†

ac

SFO pc

OVLT oc

ME

SCO

PI

NH

CP AP

Gly-Ser-Ser-Phe-Leu-Ser-Pro-Glu-His-Gln-Arg-Val-Gln-Gln-Arg-Lys-GluSer-Lys-Lys-Pro-Pro-Ala-Lys-Leu-Gln-Pro-Arg (MW = 3314.9) *Disulfide bonds between pairs of cystines that produce cyclization of the peptides are indicated by their italicized cognate Cys residues. † The serine at position 3 in ghrelin is O-octanoylated. MW, Molecular weight; pGlu, pyro-glutamyl.

Figure 7-5  Median sagittal section through the human brain shows the circumventricular organs (dark brown). Light brown areas are the optic chiasm (oc), corpus callosum (cc), anterior commissure (ac), and posterior commissure (pc). AP, area postrema; ME, median eminence; NH, neurohypophysis; OVLT, organum vasculosum of the lamina terminalis; PI, pineal gland; SFO, subfornical organ; SCO, subcommissural organ; CP, choroid plexus. (Adapted from Weindl A. Neuroendocrine aspects of circumventricular organs. In: Ganong WF, Martini L, eds. Frontiers in Neuroendocrinology, vol 3. New York, NY: Oxford University Press, 1973:3-32.)

112    Neuroendocrinology accumulation around the median eminence.51,52 Moreover, leptin receptor messenger ribonucleic acid (mRNA) and leptin-induced gene expression are densely localized in the arcuate, ventromedial, dorsomedial, and ventral premammillary hypothalamic nuclei.53 Leptin is an established mediator of body weight and neuroendocrine function that acts on several cell groups in the hypothalamus, including POMC neurons that reside in the arcuate nucleus.15,53,54 Because POMC neurons are also found embedded within the median eminence, it is likely that the median eminence is involved in conveying information from humoral factors such as leptin to key hypothalamic regulatory neurons in the medial basal hypothalamus.43

Organum Vasculosum of the Lamina Terminalis and the Subfornical Organ The OVLT and the SFO are located at the front wall of the third ventricle, the lamina terminalis. The OVLT and SFO lie, respectively, at the ventral and dorsal boundaries of the third ventricle (see Fig. 7-5).47 Because it lies at the rostral and ventral tip of the third ventricle, the OVLT is surrounded by cell groups of the preoptic region of the hypothalamus. Like other CVOs, the OVLT is composed of neurons, glial cells, and tanycytes. Axon terminals containing several neuropeptides and neurotransmitters including GnRH, somatostatin, angiotensin, dopamine, norepinephrine, serotonin, acetylcholine, oxytocin, AVP, and TRH innervate the OVLT. In the rodent, neurons that contain GnRH surround the OVLT. In addition, the OVLT in the rat brain contains estrogen receptors, and the application of estrogen or electric stimulation at this site is capable of stimulating ovulation through GnRH-containing neurons that project to the median eminence, suggesting that the region regulates sexual behavior in the rat.45 The region of the hypothalamus that immediately surrounds the OVLT regulates a diverse array of autonomic processes. However, because the OVLT is potentially involved in the maintenance of so many processes, definitive studies ascribing specific functions to the OVLT are inherently difficult. For example, lesions of the OVLT and surrounding preoptic area led to altered febrile responses after immunologic stimulation and to disruptions in fluid and electrolyte balance, blood pressure, reproduction, and thermoregulation. Large lesions of the OVLT attenuated lipopolysaccharide-induced fever.55 Consistent with this finding, it was demonstrated that receptors for prostaglandin E2 are located within and immediately surrounding the OVLT.56 Because prostaglandin E2 is thought to be an obligate endogenous pyrogen, the OVLT may be a critical regulator of febrile responses. The OVLT is also likely to be involved in sensing serum osmolality, because lesions of the OVLT attenuate AVP and oxytocin secretion in response to osmotic stimuli. In addition, hypertonic saline administration in rats induced c-Fos (a marker of neuronal activation) in OVLT neurons.57 The efferent projections of the OVLT are not well defined because of the fundamental difficulty of injecting this small structure with specific neuroanatomic tracers without contaminating surrounding preoptic nuclei. However, the neurons in the OVLT apparently have a remarkably restricted range of projections that include the PVH and SON nuclei, the dorsomedial hypothalamic nucleus, and the lateral hypothalamic area (J.K. Elmquist, J.E. Sherin, and C.B. Saper, unpublished observations). The SFO is located in the roof of the third ventricle below the fornix. This CVO critically regulates fluid homeostasis and contributes to blood pressure regulation.47

Consistent with these functions, the SFO has receptors for angiotensin II and atrial natriuretic peptide.49,58 In addition to expressing these key receptors, the SFO is thought to regulate fluid homeostasis because of its specific and massive projections to key hypothalamic regulatory sites. Notable among these are the inputs to oxytocin and AVP magnicellular neurons in the SON and PVH. Parvicellular neurons in the PVH concerned with neuroendocrine and autonomic control also receive innervation from the SFO. In addition, the SFO densely innervates the paramedian preoptic region of the hypothalamus (often known as the anteroventral third ventricular region) and other hypothalamic sites including the perifornical area of the lateral hypothalamus. A major cell group within the anteroventral third ventricular region is the median preoptic nucleus, which receives dense innervation from the SFO.59 Several neuroanatomic studies have demonstrated that the median preoptic nucleus is a major source of afferents to the magnicellular neuroendocrine neurons in the PVH and SON. In addition to the preceding neuroanatomic findings, physiologic evidence suggests that the SFO is critical in maintaining fluid balance. Simpson and Routtenberg reported that substances such as angiotensin II elicited drinking behavior when microinjected at low doses directly into the SFO.60 Later studies demonstrated that SFO neurons have electrophysiologic responses to angiotensin II.49 In addition, stimulation of the SFO elicited AVP secretion. Like the OVLT, the SFO expressed Fos after stimulation by hypertonic saline administration.57 Therefore, the SFO provides dense direct and indirect innervation to the magnicellular neuroendocrine neurons in the PVH and SON that are critical in the maintenance of fluid balance and blood pressure.

Area Postrema The area postrema lies at the caudal end of the fourth ventricle, adjacent to the nucleus of the solitary tract (see Fig. 7-5). In experimental animals such as the rat and mouse, it is a midline structure lying above the nucleus of the solitary tract.48,61 However, in humans, the area postrema is a bilateral structure. Because the area postrema overlies the nucleus of the solitary tract, it also receives direct visceral afferent input from the glossopharyngeal nerve (including the carotid sinus nerve) and the vagus nerve. In addition, the area postrema receives direct input from several hypothalamic nuclei. The efferent projections of the area postrema include projections to the nucleus of the solitary tract, the ventral lateral medulla, and the parabrachial nucleus. Consistent with its role as a sensory organ, the area postrema is enriched with receptors for several neuropeptides, including glucagon-like peptide 1 and cholecystokinin.62,63 It also contains chemosensory neurons including osmoreceptors.47 The area postrema is thought to be critical in the detection of potential toxins and can induce vomiting in response to foreign substances. In fact, the area postrema is often referred to as the chemoreceptor trigger zone.61 The best-described physiologic role of the area postrema is the coordinated control of blood pressure.47,48 The area postrema contains binding sites for angiotensin II, AVP, and atrial natriuretic peptide. Lesions of the area postrema in rats blunt the rise in blood pressure induced by angiotensin II.64 Administration of angiotensin II induces the expression of Fos in neurons of the area postrema. This area has also been hypothesized to play a role in responding to inflammatory cytokines during the acute febrile response.

Neuroendocrinology    113

Subcommissural Organ The SCO is located near the junction of the third ventricle and the cerebral aqueduct, below the posterior commissure and the pineal gland (see Fig. 7-5).47 It is composed of specialized ependymal cells that secrete a highly glycosylated protein of unknown function. The secretion of this protein leads to aggregation and formation of the so-called Reissner’s fibers.50 The glycoproteins are extruded through the aqueduct, the fourth ventricle, and the spinal cord lumen to terminate in the caudal spinal canal. In humans, intracellular secretory granules are identifiable in the SCO, but Reissner’s fibers are absent. The SCO secretion in humans is therefore presumed to be more soluble and to be absorbed directly from the CSF. Compared with other CVOs, the physiologic role of the SCO is largely unknown. Hypothesized roles for the SCO include clearance of substances from the CSF.50

PINEAL GLAND Descartes called the pineal gland the “seat of the soul.” A more contemporary, although less colorful, viewpoint is that the pineal integrates information encoded by light into coordinated secretions that underlie biologic rhythmicity.65,66 The pineal gland is both an endocrine organ and a CVO; it is derived from cells located in the roof of the third ventricle and lies above the posterior commissure near the level of the habenular complex and the sylvian aqueduct. The gland is composed of two cell types, pinealocytes and interstitial (glial-like) cells. Histologic studies suggest that the pineal gland cells are secretory in nature, and indeed the pineal is the principal source of melatonin in mammals. The pineal is an epithalamic structure and consists of primordial photoreceptive cells. The gland retains its light sensitivity in lower vertebrates such as fish and amphibians but lacks direct photosensitivity in mammals and has evolved as a strictly secretory organ in higher vertebrates. However, neuroanatomic studies have established that

light-encoded information is relayed to the pineal by a polysynaptic pathway. This series of synapses ultimately results in innervation of the gland by noradrenergic sympathetic nerve terminals that are critical regulators of melatonin production and release. Specifically, retinal ganglion cells directly innervate the SCN of the hypothalamus through the retinohypothalamic tract.67 The SCN in turn provides input to the dorsal parvicellular PVH, a key cell group in neuroendocrine and autonomic control. This pathway consists of direct and indirect intrahypothalamic projections.68,69 The PVH in turn provides direct innervation to sympathetic preganglionic neurons in the intermediolateral cell column of the thoracic regions of the spinal cord.70 Sympathetic preganglionic neurons innervate postganglionic neurons in the superior cervical ganglion71 that ultimately supply the noradrenergic innervation to the pineal (see “Hypothalamic-Pituitary Unit”). This rather circuitous pathway represents the anatomic substrate for light to regulate the secretion of melatonin. In the absence of light input, the pineal gland rhythms persist but are not entrained to the external light-dark cycle.

The Pineal Is the Source of Melatonin The predominant hormone secreted by the pineal gland is melatonin. However, the pineal contains other biogenic amines, peptides, and GABA. Pineal-derived melatonin is synthesized from tryptophan, through serotonin, with the rate-limiting step catalyzed by the enzyme arylalkylamine N-acetyltransferase (AANAT) (Fig. 7-6).72,73 HydroxyindoleO-methyltransferase (HIOMT) catalyzes the final step of melatonin synthesis. These enzymes are expressed in a pineal-specific manner; however, HIOMT is also expressed in the retina and in red blood cells. Melatonin plays a key role in regulating a myriad of circadian rhythms, and a fundamental principle of circadian biology is that the synthesis of melatonin is exquisitely controlled.65 AANAT mRNA levels, AANAT activity, and melatonin synthesis and release are regulated in a circadian fashion and are entrained by the light-dark cycle, with darkness thought to be the most important signal.66,72,73 Melatonin and AANAT levels

H H

H H

C C NH2 N H Tryptophan

H C O

1

HO N H

Tryptophan hydroxylase

OH

Aromatic-l-amino acid decarboxylase

H H H O

C C NH2 N H Serotonin

2

H C O

OH 5-Hydroxytryptophan

H H HO

C C NH2

H H

3

HO

N-Acetyltransferase + Acetyl-CoA

C C N C CH3 H H N H N-Acetylserotonin

4

HydroxyindoleO-methyltransferase + S-Adenosylmethionine

H H H O CH3O

C C N C CH3 H H N H Melatonin

Figure 7-6  Biosynthesis of melatonin from tryptophan in the pineal gland. Step 1 is catalyzed by tryptophan hydroxylase, step 2 by aromatic-L-amino acid decarboxylase, step 3 by arylalkylamine N-acetyltransferase, and step 4 by hydroxyindole-O-methyltransferase. (From Wurtman RJ, Axelrod J, Kelly DE. Biochemistry of the pineal gland. In: Wurtman RJ, Axelrod J, Kelly DE, eds. The Pineal. New York, NY: Academic Press, 1968:47-75.)

114    Neuroendocrinology are highest during the dark and decrease sharply with the onset of light. Melatonin is not stored to any significant degree; it is released into blood or CSF directly after its biosynthesis in proportion to AANAT activity. CNS control of melatonin secretion during the dark is mediated by the neuroanatomic pathway just outlined. Lack of light ultimately results in the release of norepinephrine from postganglionic sympathetic nerve terminals that act on β-adrenergic receptors in pinealocytes, resulting in an increase in adenylyl cyclase activity and synthesis of cyclic adenosine monophosphate (cAMP) from adenosine triphosphate.72 Increased levels of intracellular cAMP activate downstream signal transduction cascades, including the catalytic subunits of protein kinase A and phosphorylation of cAMP response element (CRE) binding protein. CREs have been identified in the promoter of AANAT.72,74 Therefore, light (or lack of it) acting through the sympathetic nervous system induces an increase in cAMP, representing a fundamental regulator of AANAT transcription and melatonin synthesis that ultimately results in a dramatic change of melatonin levels across the day.

Physiologic Roles of Melatonin One of the best-characterized roles of melatonin is regulation of the reproductive axis, including gonadotropin secretion75 and the timing and onset of puberty (see “Gonadotropin-Releasing Hormone and Control of the Reproductive Axis”). The potent regulation of the reproductive axis by melatonin is established in rodents and domestic animals such as the sheep. It was observed experimentally with the demonstration that removal of the pineal leads to precocious puberty. In addition, male rats exposed to constant darkness or blinded by enucleation display testicular atrophy and decreased levels of testosterone. These profound effects of gonadal involution are normalized by removal of the pineal gland.76 The physiologic significance of melatonin is probably most important in species referred to as seasonal breeders. Indeed, the role of melatonin in regulating reproductive capacity in species such as the sheep and the horse is now established. This type of reproductive strategy probably evolved to synchronize the length of day with the gestational period of the species, to ensure that the offspring are born at favorable times of the year and to maximize the viability of the young. Interestingly, although there is a strong and consistent correlation between altered melatonin secretion, day length, and seasonal breeding in diverse species, the valence of the signal can be either positive or negative, depending on the ecologic niche of each species. Despite the potent effects of day length on reproduction in these species, the exact mechanisms of melatonin regulation of GnRH release are unsettled. However, melatonin inhibits LH release from the rat pars tuberalis.75 The role of the pineal in human reproduction is even less understood.77 Earlier onset of menarche in blind women has been reported. In addition, a decline in melatonin at puberty has been described in some but not all studies. Interspecies comparative studies of melatonin’s physiologic function must be tempered by knowledge of key differences between rodent and human melatonin regulation. Significantly more light, as much as 4 log units, is required in humans to produce an equivalent nocturnal suppression of melatonin,78 and the control of AANAT is largely post-transcriptional in humans, rather than transcriptional.73

Melatonin Receptors Melatonin mediates its effects by acting on a family of G protein–coupled receptors that have been characterized by pharmacologic, neuroanatomic, and molecular approaches.65,66,73 The first member of the family, MT1 (also called Mel1a or melatonin receptor type 1A [MTNR1A]), is a high-affinity receptor that was isolated originally from Xenopus melanophores. The second, MT2 (Mel1b or MTNR1B), has approximately 60% homology with MT1. A third receptor in mammals, MT3, is not a G protein– coupled receptor but instead a high-affinity binding site on the cytosolic enzyme quinone reductase 2 that is involved in cellular detoxification; this might explain some of melatonin’s effects as an antioxidant.66,73 The mechanisms for melatonin’s effects on regulating and entraining circadian rhythms are becoming increasingly understood. For example, melatonin inhibits the activity of neurons in the SCN of the hypothalamus, the master circadian pacemaker in the mammalian brain.65,79,80 Melatonin can entrain several mammalian circadian rhythms, probably by inhibition of neurons in the SCN. Neuroanatomic evidence suggests that many of the effects of melatonin on circadian rhythms involve actions on MT1 receptors, in that the distribution of MT1 mRNA overlaps with radiolabeled melatonin-binding sites in the relevant brain regions. These sites include the SCN, the retina, and the pars tuberalis of the adenohypophysis. The MT2 receptor is also expressed in retina and brain, particularly in the SCN, but evidently at much lower levels.65,73,79 Genetic studies in mice have also helped to illuminate the relative roles of each melatonin receptor in mediating the effects of this hormone. Targeted deletion (knockout) of the MT1 but not the MT2 receptor abolished the ability of melatonin to inhibit the activity of SCN neurons.80,81 Several studies have suggested that inhibition of SCN neurons by melatonin is of great physiologic significance. For example, Reppert and colleagues proposed that elevations of melatonin at night decrease the responsiveness of the SCN to activity-related stimuli that could result in phase shifts.80 As noted, light potently inhibits melatonin synthesis and release. Therefore, melatonin may underlie the mechanism by which light induces phase shifts. However, it should be noted that lack of the MT1 gene does not block the ability of melatonin to induce phase shifts. These unexpected and somewhat confusing results have resulted in the hypothesis that MT2 is involved in melatonin-induced phase shifts, because this receptor is expressed in the SCN in human brain.73

Melatonin Therapy in Humans Melatonin is purported to exert multiple beneficial functions that include slowing or reversing the progression of aging, protecting against ischemic damage after vascular reperfusion, and enhancing immune function.66,73 However, the most studied and established role of melatonin in humans is that of phase shifting and resetting circadian rhythms. In this context, melatonin has been used to treat jet lag and may be effective in treating circadian-based sleep disorders.82 In addition, melatonin administration has been shown to regulate sleep in humans. Specifically, melatonin has a hypnotic effect at relatively low doses. Melatonin therapy has also been suggested as a way to treat seasonal affective disorders. However, two recent metaanalyses of the published reports on melatonin for treatment of either primary or secondary sleep disorders concluded that there is limited evidence for significant

Neuroendocrinology    115

clinical efficacy, although melatonin is safe with shortterm use (≤3 months).83,84 Because melatonin is now available over the counter and without a prescription throughout the United States, it is important that further controlled clinical studies be conducted to assess fully the therapeutic potential and safety of long-term melatonin use in humans.

HYPOPHYSEOTROPIC HORMONES AND NEUROENDOCRINE AXES With the demonstration by the first half of the 1900s that pituitary secretion is controlled by hypothalamic hormones released into the portal circulation, the race was on to identify the hypothalamic releasing factors. The search for hypothalamic neurohormones with anterior pituitary– regulating properties focused on extracts of stalk median eminence, neural lobe, and hypothalamus from sheep and pigs. To give some idea of the herculean nature of this effort, approximately 250,000 hypothalamic fragments were required to purify and characterize the first such factor, TRH.9 Such hypophyseotropic substances were initially called releasing factors but are now more commonly called releasing hormones. All of the principal hypothalamic-pituitary regulating hormones are peptides, with the notable exception of dopamine, which is a biogenic amine and the major prolactin-inhibiting factor (PIF; see later discussion and Table 7-2). All are available for clinical investigations or diagnostic tests, and therapeutic analogues for dopamine, GnRH, and somatostatin are widely prescribed. In addition to regulating hormone release, some hypophyseotropic factors control pituitary cell differentiation and proliferation and hormone synthesis. Some act on more than one pituitary hormone. For example, TRH is a potent releaser of prolactin (PRL) and of TSH, and under some circumstances it releases corticotropin (ACTH) and growth hormone (GH). GnRH releases both LH and folliclestimulating hormone (FSH). Somatostatin inhibits the secretion of GH, TSH, and a wide variety of nonpituitary hormones. The principal inhibitor of PRL secretion, dopamine, also inhibits secretion of TSH, gonadotropin, and, under certain conditions, GH. Dual control is exerted by the interaction of inhibitory and stimulatory hypothalamic hormones. For example, somatostatin interacts with GHRH and TRH to control secretion of GH and TSH, respectively, and dopamine interacts with prolactinreleasing factors (PRFs) to regulate PRL secretion. Some hypothalamic hormones act synergistically; for example, CRH and AVP cooperatively regulate the release of pituitary ACTH. Secretion of the releasing hormones in turn is regulated by neurotransmitters and neuropeptides released by a complex array of neurons synapsing with hypophyseotropic neurons. Control of secretion is also exerted through feedback control by hormones such as glucocorticoids, gonadal steroids, thyroid hormone, anterior pituitary hormones (short-loop feedback control), and hypophyseotropic factors themselves (ultrashort-loop feedback control). The distribution of the hypophyseotropic hormones is not limited to the hypothalamus. Most are produced in nonhypophyseotropic hypothalamic neurons, in extrahypothalamic regions of the brain, and in peripheral organs where they mediate functions unrelated to pituitary regulation (e.g., effects on behavior or homeostasis). Most of the peptides, hormones, and neurotransmitters involved in the regulation of hypothalamic-pituitary control transduce

their signals through members of the extensive G protein– coupled receptor family (Table 7-3).

Feedback Concepts in Neuroendocrinology To understand the regulation of each hypothalamicpituitary–target organ axis, it is important to understand some basic concepts of homeostatic systems. A simplified account of feedback control in relation to neuroendocrine regulation is presented here.85-87 Hormonal systems form part of a feedback loop in which the controlled variable (usually the blood hormone level or some biochemical surrogate of the hormone) determines the rate of secretion of the hormone. In negative feedback systems, the controlled variable inhibits hormone output, and in positive feedback control systems it increases hormone secretion. Both negative and positive endocrine feedback control systems can be part of a closed loop, in which regulation is entirely restricted to the interacting regulatory glands, or an open loop, in which the nervous system influences the feedback loop. All pituitary feedback systems have nervous system inputs that either alter the set point of the feedback control system or introduce open-loop elements that can influence or override the closed-loop control elements. In engineering formulations of feedback, three types of controlled variables can be identified: a sensing element that detects the concentration of the controlled variable, a reference input that defines the proper control levels, and an error signal that determines the output of the system. The reference input is the set point of the system. Hormonal feedback control systems resemble engineering systems in that the concentration of the hormone in the blood (or some function of the hormone) regulates the output of the controlling gland. However, hormonal feedback differs from engineering systems in that the sensor element and the reference input element are not readily distinguishable. The set point of the controlled variable is determined by a complex cascade beginning with the kinetics of binding to a receptor and the activities of successive intermediate messengers. Sophisticated models incorporating control elements, compartmental analysis, and hormone production and clearance rates exist for many systems. This sort of modeling, which is applied to developmental programming, intracellular signaling cascades, and neural circuits in addition to endocrine feedback systems, is commonly referred to as systems biology.88

Endocrine Rhythms Virtually all functions of living animals (regardless of their position on the evolutionary scale) are subject to periodic or cyclic changes, many of which are influenced primarily by the nervous system (Table 7-4).89-92 Most periodic changes are free-running; that is, they are intrinsic to the organism, independent of the environment, and driven by a biologic “clock.” Most free-running rhythms are coordinated (entrained) by external signals (cues), such as light-dark changes, meal patterns, cycles of the lunar periods, or the ratio of daylength to night-length. External signals of this type (zeitgeber, or “time givers”) do not bring about the rhythm but provide the synchronizing time cue. Many endogenous rhythms have a period of approximately 24 hours. Circadian changes follow an intrinsic program that is about 24 hours long, whereas diurnal rhythms can be either circadian or dependent on shifts in light and dark. Rhythms that occur more frequently than once a day are ultradian. Infradian rhythms have a period longer than 1 day, as in

116    Neuroendocrinology TABLE 7-3 

Receptors for Neurotransmitters and Neuropeptides Involved in Hypothalamic-Pituitary Control and Neuroendocrine Homeostasis Group and Ligand

Receptor Family

Receptor Protein*

Receptor Gene

Mode of Action†

α1-Adrenoreceptors

ADA1A (α1A) ADA1B (α1B) ADA1D (α1D) ADA2A (α2A) ADA2B (α2B) ADA2C (α2C) ADRB1 (β1) ADRB2 (β2) ADRB3 (β3) 5HT1A (5HT1A-α) 5HT1B (5HT1D-β) 5HT1D (5HT1D-α) 5HT1E 5HT2A 5HT2B 5HT2C (5HT1C) 5HT3 Subunit genes 5HT4R DRD1 (D1-R, D1A) DRD2 (D2-R) DRD3 (D3-R) DRD4 (D4-R, D2C) DRD5 (D5-R, D1B) HRH1 (H1-R) HRH2 (H2-R) HRH3 (H3-R) MT1RA (Mel1AR, MT1) MT1RB (Mel1BR, MT2) MT3 (quinone reductase 2) TAAR1 (TaR-1) ACM1 (M1) ACM2 (M2) ACM3 (M3) ACM4 (M4) ACM5 (M5) ACHA-P, ACH1-7 Subunit genes NMDA (NR1, NR2A-D) NMZ1 subunit gene AMPA (GluR1-4) GRIA1 subunit gene Kainate (GluR5-7, KA-1/2) LK1 subunit gene MGR1 (mGluR1) MGR2 (mGluR2) MGR3 (mGluR3) MGR4 (mGluR4) MGR5 (mGluR5) MGR6 (mGluR6) MGR7 (mGluR7) GAA-E (GABA-A-R) GAA1 (α1) subunit gene GABR1 (GABA-B-R1) GABR2 (GABA-B-R2)

ADRA1A ADRA1B ADRA1D ADRA2A ADRA2B ADRA2C ADRB1 ADRB2 ADRB3 HTR1A HTR1B HTR1D HTR1E HTR2A HTR2B HTR2C Pentamer HTR3A, HTR3B HTR4 DRD1 DRD2 DRD3 DRD4 DRD5 HRH1 HRH2 HRH3 MTNR1A MTNR1B NQO2 TAAR1 CHRM1 CHRM2 CHRM3 CHRM4 CHRM5 Pentamer CHRNA, CHRNB Oligomer GRIN1 (NMDAR1) Oligomer GRIA1 (GLUR1) Oligomer GRIK1 (GLUR5) GRM1 GRM2 GRM3 GRM4 GRM5 GRM6 GRM7 Pentamer GABRA1 GABBR1 GABBR2

7-TM, Gq/11 7-TM, Gq/11 7-TM, Gq/11 7-TM, Gi/o 7-TM, Gi/o 7-TM, Gi/o 7-TM, GS 7-TM, GS 7-TM, GS 7-TM, Gi/o 7-TM, Gi/o 7-TM, Gi/o 7-TM, Gi/o 7-TM, Gq/11 7-TM, Gq/11 7-TM, Gq/11 Cation flux

Classic Neurotransmitters Catecholamines (norepinephrine, epinephrine)

α2-Adrenoreceptors

β-Adrenoreceptors

Serotonin (5-HT)

5-HT1 receptors

5-HT2 receptors

5-HT3 receptors

Dopamine

5-HT4 receptors Dopamine receptors

Histamine

Histamine receptors

Melatonin

Melatonin receptors

Trace amines Acetylcholine

Trace amine receptor Muscarinic receptors

Nicotinic receptors Glutamate

Ionotropic receptors

Metabotropic receptors

γ-Aminobutyric acid (GABA)

Ionotropic Heterodimeric

7-TM, GS 7-TM, GS 7-TM, Gi/o 7-TM, Gi/o 7-TM, Gi/o 7-TM, GS 7-TM, Gq/11 7-TM, GS 7-TM, Gi/o 7-TM, Gi/o, PLC-β 7-TM, Gi/o, Gq/11 Cytosolic enzyme 7-TM, GS 7-TM, Gq/11 7-TM, Gq/11 7-TM, Gq/11 7-TM, Gi/o 7-TM, Gq/11 Cation flux Cation flux Cation flux Cation flux 7-TM, Gq/11 7-TM, Gi/o 7-TM, Gi/o 7-TM, Gi/o 7-TM, Gq/11 7-TM, Gi/o 7-TM, Gi/o [Cl−] ion flux 7-TM, Gi/o 7-TM, Gi/o

Neuroendocrinology    117 TABLE 7-3 

Receptors for Neurotransmitters and Neuropeptides Involved in Hypothalamic-Pituitary Control and Neuroendocrine Homeostasis (Continued) Receptor Family

Receptor Protein*

Receptor Gene

Mode of Action†

Neurohypophyseal hormones Vasopressin (AVP)

Vasopressin receptors

Oxytocin

Oxytocin receptor

V1AR (V1a) V1BR (V1b, V3) V2R (ADH-R) OXYR (OT-R)

AVPR1A AVPR1B AVPR2 OXTR

7-TM, Gq/11 7-TM, Gq/11 7-TM, GS 7-TM, Gq/11

Hypophyseotropic hormones Thyrotropin-releasing hormone (TRH) Growth hormone–releasing hormone (GHRH) GHRP/Ghrelin Gonadotropin-releasing hormone (GnRH) Corticotropin-releasing hormone (CRH)/Urocortin

TRH receptor GHRH receptor GHS receptor GnRH receptor CRH receptors

Somatostatin/Cortistatin

Somatostatin receptors

TRFR (TRH-R) GHRHR (GRFR) GHSR (GHRP-R) GNRHR (GnRH-R) CRFR1 (CRH-R1) CRFR2 (CRH-R2) SSR1 (SS1R, SRIF-2) SSR2 (SS2R, SRIF-1) SSR3 (SS3R, SSR-28) SSR4 (SS4R) SSR5 (SS5R)

TRHR GHRHR GHSR GNRHR CRHR1 CRHR2 SSTR1 SSTR2 SSTR3 SSTR4 SSTR5

7-TM, Gq/11 7-TM, GS 7-TM, Gq/11 7-TM, Gq/11 7-TM, GS 7-TM, GS 7-TM, Gi/o 7-TM, Gi/o 7-TM, Gi/o 7-TM, Gi/o 7-TM, Gi/o

Endogenous opioid peptides β-Endorphin Enkephalin Dynorphin Nociceptin/OFQ

Mu opioid receptor Delta opioid receptor Kappa opioid receptor OFQ opioid receptor

OPRM (µ, MOR-1) OPRD (δ, DOR-1) OPRK (κ, KOR-1) OPRX (KOR-3)

OPRM1 OPRD1 OPRK1 OPRL1

7-TM, Gi/o 7-TM, Gi/o 7-TM, Gi/o 7-TM, Gi/o

Melanocortin peptides Melanocyte–stimulating hormone (MSH) Corticotropin (ACTH) γ-MSH, α-MSH α-MSH, β-MSH α-MSH

MSH receptor ACTH receptor Melanocortin receptor 3 Melanocortin receptor 4 Melanocortin receptor 5

MSHR (MC1-R) ACTHR (MC2-R) MC3R (MC3-R) MC4R (MC4-R) MC5R (MC5-R)

MC1R MC2R MC3R MC4R MC5R

7-TM, GS 7-TM, GS 7-TM, GS 7-TM, GS 7-TM, GS

Neurokinin receptors

NK1R (SPR) NK2R (SKR) NK3R (NKR)

TACR1 TACR2 TACR3

7-TM, Gi/o 7-TM, Gi/o 7-TM, Gi/o

Vasoactive peptides Angiotensin II

Angiotensin receptors

Atrial natriuretic peptide (ANP)

ANP receptors

Endothelin

Endothelin receptors

AGTR1 (AT1) AGTR2 (AT2) ANPRA (NPR-A) ANPRB (NPR-B) ENDRA (ETA-R) ENDRB (ETB-R)

AGTR1 AGTR2 NPR1 NPR2 EDNRA EDNRB

7-TM, Gq/11 7-TM, Gi/o cGMP, 1-TM cGMP, 1-TM 7-TM, Gq/11 7-TM, Gq/11

Miscellaneous neuropeptides CART Orexin/hypocretin

No receptor identified Orexin receptors

Melanin-concentrating hormone (MCH) Prolactin-releasing peptide (PrRP) Kisspeptins/Metastin Neuromedin U

MCH receptor PrRP receptor Kisspeptin receptor Neuromedin receptors

Neurotensin PACAP Vasoactive intestinal peptide (VIP)

Neurotensin receptor PACAP receptor VIP receptors

Galanin/GALP

Galanin receptors

OX1R (HCRTR-1) OX2R (HCRTR-2) MCHR1 (GPCR24) PRLHR (GPCR10) KISSR (GPCR54) NMUR1 (GPCR66) NMUR2 NTR1 (NTRH) PACR (PACAP-R-1) VIPR1 (PACAP-R-2) VIPR2 (PACAP-R-3) GALR1 (GAL1-R) GALR2 (GAL2-R) GALR3 (GAL3-R)

HCRTR1 HCRTR2 MCHR1 PRLHR KISS1R NMUR1 NMUR2 NTSR1 ADCYAR1R1 VIPR1 VIPR3 GALR1 GALR2 GALR3

7-TM, Gi/o 7-TM, many 7-TM, many 7-TM, Gi/q 7-TM, Gi/o/q 7-TM, Gq/11 7-TM, Gq/11 7-TM, Gq/11 7-TM, Gq/11 7-TM, GS 7-TM, GS 7-TM, GS 7-TM, Gi/o 7-TM, Gi/o 7-TM, Gi/o

Group and Ligand Neuropeptides

Tachykinins (neurokinins) Substance P Substance K Neurokinin B (NKB)

Table continued on following page

118    Neuroendocrinology TABLE 7-3 

Receptors for Neurotransmitters and Neuropeptides Involved in Hypothalamic-Pituitary Control and Neuroendocrine Homeostasis (Continued) Group and Ligand

Receptor Family

Receptor Protein*

Receptor Gene

Mode of Action†

Glucagon-like peptide (GLP) Cholecystokinin (CCK)/Gastrin

GLP receptor CCK receptors

Neuropeptide Y (NPY) Peptide YY (PYY3-32) Pancreatic polypeptide (PP) Neuropeptide Y

NPY/PYY/PP receptors

GLP1R CCKAR (CCK1-R) GASR (CCK2-R) NPY1R (NPY-Y1) NPY2R (NPY-Y2) NPY4R (PP1) NPY5R (NPY-Y5)

GLP1R CCKAR CCKBR NPY1R NPY2R PPYR1 NPY5R

7-TM, GS 7-TM, Gq/11 7-TM, Gq/11 7-TM, Gi/o 7-TM, Gi/o 7-TM, Gi/o 7-TM, Gi/o

Cannabinoid receptor

CNR1 (CB1)

CNR1

7-TM, Gi/o

Other Cannabinoid

*Receptors cited are human. Swiss-Prot identifiers and alternative names (in parentheses) are provided for each receptor and were obtained with the use of the GPCRDB information system (available at http://www.gpcr.org/7tm/) described in Horn F, Bettler E, Oliveira L, et al. GPCRDB information system for G proteincoupled receptors. Nucleic Acids Res. 2003;31:294-297. † The mode of action designation is oversimplified. It is common for 7-TM GPCRs to interact with multiple different G-protein complexes depending on the specific cell. Gi/o, GPCR coupled to the Gi/o family, inhibits adenylyl cyclase and decreases intracellular cAMP, opens K+ channels and closes Ca2+ channels; Gq/11, GPCR coupled to the Gq/11 family, stimulates the phosphoinositol cascade; GS, GPCR coupled to the GS family, stimulates adenylyl cyclase and increases intracellular cAMP; PLC-β, GPCR coupled to G protein that activates phospholipase Cβ (PLC-β); cGMP, guanylate cyclase activity intrinsic to these 1 transmembrane pass receptors. AMPA, α-amino-3-hydroxy-5-methyl-4-isoxazdeproprionic acid; cAMP, cyclic adenosine monophosphate; CART, cocaine- and amphetamine-responsive transcript; GALP, galanin-like peptide; (GPCR) G protein–coupled receptor; GHS, growth hormone secretagogue; NMDA, N-methyl-D-aspartate; OFQ, orphanin FQ; PACAP, pituitary adenylyl cyclase activating peptide; TM, transmembrane.

the approximately 27-day human menstrual cycle and the yearly breeding patterns of some animals. Most endocrine rhythms are circadian (Fig. 7-7). The secretion of GH and PRL in humans is maximal shortly after the onset of sleep, and that of cortisol is maximal between 2 a.m. and 4 a.m. TSH secretion is lowest in the morning, between 9 a.m. and 12 noon, and maximal between 8 p.m. and midnight. Gonadotropin secretion in adolescents is increased at night. Superimposed on the circadian cycle are ultradian bursts of hormone secretion. LH secretion during adolescence is characterized by rapid, high-amplitude pulsations at night, whereas in sexually

TABLE 7-4 

Terms Used to Describe Cyclic Endocrine Phenomena Period Circadian Diurnal Ultradian Infradian Mean Range Nadir Acrophase Zeitgeber Entrainment Phase shift Intrinsic clock

Length of the cycle About a day (24 hr) Exactly a day Less than a day (i.e., minutes or hours) Longer than a day (i.e., month or year) Arithmetic mean of all values within a cycle Difference between the highest and lowest values Minimal level (inferred from mathematical curve fitting calculations) Time of maximal levels (inferred from curve fitting) “Time-giver” (German); the external cue, usually the light-dark cycle that synchronizes endogenous rhythms The process by which an endogenous rhythm is regulated by a zeitgeber Induced change in an endogenous rhythm Neural structures that possess intrinsic capacity for spontaneous rhythms; for circadian rhythms, these are located in the suprachiasmatic nucleus

(Adapted from Van Cauter E,Turek FW. Endocrine and other biological rhythms. In DeGroot LJ, ed. Endocrinology, 3rd ed. Philadelphia, PA: Saunders, 1995:2497-2548).

mature individuals secretory episodes are lower in amplitude and occur throughout the 24 hours. GH, ACTH, and PRL are also secreted in brief, fairly regular pulses. The short-term fluctuations in hormonal secretion have important functional significance. In the case of LH, the normal endogenous rhythm of pituitary secretion reflects the pulsatile release of GnRH. The period of approximately 90 minutes between LH peaks corresponds to the optimal timing of GnRH pulses to induce maximal pituitary stimulation. Episodic secretion of GH also enhances its biopotency, but for many rhythms, the function is not clear. Most homeostatic activities are rhythmic, including body temperature, water balance, blood volume, sleep, and activity.93,94 Assessment of endocrine function must take into account the variability of hormone levels in the blood. Appropriately obtained samples at different times of day or night may provide useful dynamic indicators of hypothalamic-pituitary function. For example, the loss of diurnal rhythm of GH and ACTH secretion may be an early sign of hypothalamic dysfunction. The optimal timing for administration of glucocorticoids to suppress ACTH secretion (e.g., in therapy for congenital adrenal hyperplasia) must take into account the varying suppressibility of the axis at different times of day. The best understood neural structures responsible for circadian rhythms are the SCN, paired structures in the anterior hypothalamus above the optic chiasm.90,94 In addition to the retinohypothalamic projection from the retina described earlier, the SCN receives neuronal input from many nuclei. Individual cells of the SCN have an intrinsic capacity to oscillate in a circadian pattern,95 and the nucleus is organized to permit many reciprocal neuron-neuron interactions through direct synaptic contacts. It is especially rich in neuropeptides, including somatostatin, VIP, NPY, and neurotensin. Microinjections of pancreatic polypeptide into the SCN reset the timing cycle of some circadian rhythms in hamsters. The SCN also responds to the pineal hormone melatonin through melatonin receptors.66,73 Studies have indicated that intrinsic pacemaker

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D

2

Plasma melatonin (pmol/L)

1

0

-1

400 300 200 100 0

-2

Leptin (% change from 0800 h)

C

E

600

Serum TSH (mU/L)

3.5

400

200

0

3.0 2.5 2.0 1.5 1.0 0.5 0.0

F

140

30 120 Peripheral LH conc. (ng/mL)

Plasma cortisol (nmol/L)

B

100 80 60 0800 1200 1600 2000 2400 0400 0800 1200 1600 2000 24-hour clock time

20 8 6 4 2 0

10 0

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Normalized CSF IR-hCRH (SD)

A

0800 1200 1600 2000 2400 0400 0800 1200 1600 2000 24-hour clock time

Figure 7-7 Diurnal rhythms of secretion. Secretion of human corticotropin-releasing hormone (hCRH). A, Cortisol; B, leptin; C, melatonin; D, and E, thyrotropin (TSH). Vertical lines indicate standard deviations (SD). F, Relationship between secretion of gonadotropin-releasing hormone (GnRH; open circles) and that of luteinizing hormone (LH; filled circles) in sheep. CSF, cerebrospinal fluid; IR, immunoreactive. (From Kling MA, DeBellis MD, O’Rourke DK, et al. Diurnal variation of cerebrospinal fluid immunoreactive corticotropin-releasing hormone levels in healthy volunteers. J Clin Endocrinol Metab. 1994;79:233-239, Fig. 3; van Coevorden A, Mockel J, Laurent E, et al. Neuroendocrine rhythms and sleep in aging men. Am J Physiol. 1991;260:E651-E661, Fig. 1A and C; Sinha MK, Ohannesian JP, Heiman ML, et al. Nocturnal rise of leptin in lean, obese, and non-insulin-dependent diabetes mellitus subjects. J Clin Invest. 1996;97:1344-1347, Fig. 2; Brabant G, Prank K, Ranft U, et al. Physiological regulation of circadian and pulsatile thyrotropin secretion in normal man and woman. J Clin Endocrinol Metab. 1990;70:403-409, Fig. 2B; and Clarke IJ, Cummins JT. The temporal relationship between gonadotropin releasing hormone [GnRH] and luteinizing hormone [LH] secretion in ovariectomized ewes. Endocrinology. 1982;111:1737-1739, Fig. 2A.)

function is not unique to neurons of the SCN; circadian oscillators are also found in many peripheral tissues.94 Metabolic changes in the SCN, such as increased uptake of 2-deoxyglucose and an increased level of VIP, accompany circadian rhythms. This nucleus projects to the pineal gland indirectly via the PVH and the autonomic nervous system (see earlier discussion) and regulates its activity.90 However, the bulk of SCN outflow occurs in a trunk that courses dorsolaterally through the ventral subparaventricular zone and terminates in the dorsal medial hypothalamic nucleus. Polysynaptic pathways involving these latter structures are responsible for the actions of the SCN in producing the circadian rhythms in thermoregulation, glucocorticoid secretion, sleep, arousal, and feeding.90 Circadian rhythms during fetal life are regulated by maternal circadian rhythms.96 Circadian changes can be

detected 2 to 3 days before birth, and SCN from fetuses of this age display spontaneous rhythmicity in vitro. Maternal regulation of fetal circadian rhythms may be mediated by circulating melatonin or by cyclic changes in the food intake of the mother. The timing of the circadian pacemaker can be shifted in humans by administration of triazolam (a short-acting benzodiazepine) or melatonin (described earlier) or by altered patterns of intense illumination.78

Thyrotropin-Releasing Hormone Chemistry and Evolution TRH, the smallest known peptide hypophyseotropic hormone, is the tripeptide pyroGlu-His-Pro-NH2. Six copies

GRE

Stat Sp1

+5 0

-22

5

120    Neuroendocrinology

TRE TRE

TRE/CRE CRE

Exon 1

TATA

Exon 3

Exon 2 Intron 1

Intron 2

200bp

Poly A 5'-UTR

3'-UTR

Signal peptide

100bp

PC1/PC2 6x

(Gln-His-Pro-Gly-Lys/Arg-Lys)

Gln-His-Pro-Gly. This peptide is then amidated at the carboxy terminus by peptidylglycine α-amidating monooxygenase (PAM), with Gly acting as the amide donor. The amino-terminal pyro-glutamate (pyroGlu) residue results from cyclization of the Gln. TRH is a phylogenetically ancient peptide that has been isolated from primitive vertebrates such as the lamprey and even from invertebrates such as the snail. TRH is widely expressed in both the CNS and periphery in amphibians, reptiles, and fishes but does not stimulate TSH release in these poikilothermic vertebrates. Therefore, TRH has multiple peripheral and central activities and was co-opted as a hypophyseotropic factor midway during the evolution of vertebrates, perhaps specifically as a factor needed for coordinated regulation of temperature homeostasis. Although the TRH tripeptide is the only established hormone encoded within its large prohormone, rat pro-TRH yields seven additional peptides that have unique tissue distributions.99 Several biologic activities of these peptides have been observed. The fragment pro-TRH(160-169) may be a hypophyseotropic factor because it is released from hypothalamic slices and potentiates the TSH-releasing effects of TRH. The pro-TRH(178-199) is also released from the median eminence and appears to inhibit ACTH release.

CPE

Effects on the Pituitary Gland and Mechanism of Action

Gln-His-Pro-Gly PAM Gln-His-Pro-NH2 cyclization of Gln

O O

C

O NH

N

CH

C N

CH2

H N

N

C H

O

NH2

(pyro) Glu-His-Pro-NH2 Figure 7-8  Structure of the human thyrotropin-releasing hormone (TRH) gene, complementary DNA, and prohormone, showing six repeats of the TRH peptide sequence encoded within exon 3. CPE, carboxypeptidase E; CRE, cyclic adenosine monophosphate (cAMP) response element; GRE, glucocorticoid response element; PAM, peptidylglycine α-amidating monooxygenase; PC1/PC2, prohormone convertases 1 and 2; Sp1, specificity protein 1 binding sequence; Stat, signal transducer and activator of transcription binding sequence; TATA, Goldstein-Hogness box involved in binding RNA polymerase; TRE, thyroid hormone response element; UTR, untranslated. (Adapted from data in Yamada M, Radovick S, Wondisford FE, et al. Cloning and structure of human genomic DNA and hypothalamic cDNA encoding human preprothyrotropin-releasing hormone. Mol Endocrinol. 1990;4:551-556.)

of the TRH peptide sequence are encoded within the human TRH pre-prohormone gene (Fig. 7-8).97 The rat pro-TRH precursor contains five TRH peptide repeats flanked by dibasic residues (Lys-Arg or Arg-Arg), along with seven or more non-TRH peptides.98 Two prohormone convertases, PC1 and PC2, cleave the TRH tripeptides at the dibasic residues as the prohormone molecule transits the regulated secretory pathway. Carboxypeptidase E then removes the dibasic residues, leaving the sequence

After intravenous injection of TRH in humans, serum TSH levels rise within a few minutes,100 followed by a rise in serum triiodothyronine (T3) levels. There is an increase in thyroxine (T4) release as well, but a change in blood levels of T4 is usually not demonstrable because the pool of circulating T4 (most of which is bound to carrier proteins) is so large. The clinical applications of TRH testing are covered later in this chapter and in Chapter 11. TRH action on the pituitary is blocked by previous treatment with thyroid hormone, which is a crucial element in the feedback control of pituitary TSH secretion. TRH is also a potent PRF.100 The time course of response of blood PRL levels to TRH, the dose-response characteristics, and the suppression by thyroid hormone pretreatment (all of which parallel changes in TSH secretion) suggest that TRH may be involved in the regulation of PRL secretion. Moreover, TRH is present in the hypophyseal-portal blood of lactating rats. However, it is unlikely to be a physiologic regulator of PRL secretion, because the PRL response to nursing in humans is unaccompanied by changes in plasma TSH levels,101 and mice lacking TRH have normal lactotrophs and normal basal PRL secretion.102 Nevertheless, TRH may occasionally cause hyperprolactinemia (with or without galactorrhea) in patients with hypothyroidism. In normal individuals, TRH has no influence on the secretion of pituitary hormones other than TSH and PRL, but it enhances the release of GH in patients with acromegaly and of ACTH in some patients with Cushing’s disease. Furthermore, prolonged stimulation of the normal pituitary with GHRH can sensitize it to the GH-releasing effects of TRH. TRH also causes the release of GH in some patients with uremia, hepatic disease, anorexia nervosa, or psychotic depression and in children with hypothyroidism.100 TRH inhibits sleep-induced GH release through its actions in the CNS (see later discussion). Stimulatory effects of TRH are initiated by binding of the peptide to specific receptors on the plasma membrane of the thyrotroph.103 Thyroid hormone and somatostatin antagonize the effects of TRH but do not interfere with its binding. TRH action is mediated mainly through hydrolysis

Neuroendocrinology    121

of phosphatidylinositol, with phosphorylation of key protein kinases and an increase in intracellular free calcium (Ca2+) as the crucial steps in postreceptor activation (see Chapter 5).104 TRH effects can be mimicked by exposure to a Ca2+ ionophore and are partially abolished by a Ca2+-free medium. TRH stimulates the formation of mRNAs coding for TSH and PRL, in addition to regulating their secretion and stimulates the mitogenesis of thyrotrophs. TRH is degraded to acid TRH and to the dipeptide histidylprolineamide, which cyclizes nonenzymatically to histidylproline diketopiperazine (cyclic His-Pro). Acid TRH has some behavioral effects in rats that are similar to those of TRH but no other proven actions. Cyclic His-Pro is reported to act as a PRF and to have other neural effects, including reversal of ethanol-induced sleep (TRH is also effective in this system), elevation of brain cyclic guanosine monophosphate levels, an increase in stereotypical behavior, modification of body temperature, and inhibition of eating behavior. Some of the effects of TRH may be mediated through cyclic His-Pro, but the fact that cyclic His-Pro is abundant in some areas and is not proportional to the amount of TRH suggests that the peptide may not be derived solely from TRH. This latter assertion appears to be confirmed by the detection of substantial amounts of the dipeptide in brains of TRH knockout mice.102

Extrapituitary Function TRH is present in virtually all parts of the brain: cerebral cortex, CVOs, neurohypophysis, pineal gland, and spinal cord.105 TRH is also found in pancreatic islet cells and in the gastrointestinal tract. Although it exists in low concentration, the total amount in extrahypothalamic tissues exceeds the amount in the hypothalamus. The extensive extrahypothalamic distribution of TRH, its localization in nerve endings, and the presence of TRH receptors in brain tissue suggest that TRH serves as a neurotransmitter or neuromodulator outside the hypothalamus.106 TRH is a general stimulant and induces hyperthermia on intracerebroventricular injection, suggesting a role in central thermoregulation.105 Studies in TRH knockout mice may further clarify the nonhypophyseotropic actions of TRH.102

Clinical Applications The use of TRH for the diagnosis of hyperthyroidism has become less common since the development of ultrasensitive assays for TSH100 (see Chapter 11). Use of TRH to discriminate between hypothalamic and pituitary causes of TSH deficiency has also declined because of the test’s poor specificity,100 but the application of ultrasensitive assays in conjunction with the TRH test has not been fully evaluated. TRH testing is also not of value in the differential diagnosis of causes of hyperprolactinemia, but it is useful for the demonstration of residual abnormal somatotropinsecreting cells in acromegalic patients who release GH in response to TRH before treatment. Studies of the effect of TRH on depression have shown inconsistent results, possibly because of poor blood-brain barrier penetration.105 Intrathecal administration of TRH may improve responses in depressed patients, but its clinical utility is unknown.107 Although a role for TRH in depression is not established, many depressed patients have a blunted TSH response to TRH, and changes in TRH responsiveness correlate with the clinical course. The mechanism by which blunting occurs is unknown. TRH has been evaluated for the treatment of diverse neurobiologic disorders (for review, see Gary and colleagues105) including spinal muscle atrophy and

amyotrophic lateral sclerosis; transient improvement in strength was reported in both disorders, but the combined experience at many centers using a variety of treatment protocols including long-term intrathecal administration failed to confirm efficacy. TRH administration reduces the severity of experimentally induced spinal and ischemic shock; preliminary studies in humans suggest that TRH treatment may improve recovery after spinal cord injury and head trauma. TRH has been used to treat children with neurologic disorders including West’s syndrome, LennoxGastaut syndrome, early infantile epileptic encephalopathy, and intractable epilepsy.108 TRH has been proposed to be an analeptic agent. Sleeping or drug-sedated animals were awakened by the administration of TRH, TRH reportedly reversed sedative effects of ethanol in humans, and TRH is said to have awakened a patient with a profound sleep disorder caused by a hypothalamic and midbrain eosinophilic granuloma.105

Regulation of Thyrotropin Release The secretion of TSH is regulated by two interacting elements: negative feedback by thyroid hormone and openloop neural control by hypothalamic hypophyseotropic factors (Fig. 7-9). TSH secretion is also modified by other hormones, including estrogens, glucocorticoids, and possibly GH, and it is inhibited by cytokines in the pituitary and hypothalamus.100,109 Aspects of the pituitary-thyroid axis are considered further in Chapter 11.

Feedback Control: Pituitary-Thyroid Axis In the context of a feedback system, the level of thyroid hormone or of its unbound fraction in blood is the controlled variable, and the set point is the normal resting level of plasma thyroid hormone. Secretion of TSH is inversely regulated by the level of thyroid hormone so that deviations from the set point lead to appropriate changes in the rate of TSH secretion (Fig. 7-10). Factors that determine the rate of TSH secretion required to maintain a given level of thyroid hormone include the rate at which TSH and thyroid hormone disappear from the blood (turnover rate) and the rate at which T4 is converted to its more active form, T3. Thyroid hormones act on both the pituitary and the hypothalamus. Feedback control of the pituitary by thyroid hormone is remarkably precise.109a,109b Administration of small doses of T3 and T4 inhibited the TSH response to TRH, and barely detectable decreases in plasma thyroid hormone levels were sufficient to sensitize the pituitary to TRH. TRH stimulates TSH secretion within a few minutes through its action on a membrane receptor, whereas thyroid hormone actions, mediated by intranuclear receptors, require several hours to take effect (see Chapter 11). The secretion of hypothalamic TRH is also regulated by thyroid hormone feedback. Systemic injections of T3 or implantations of tiny T3 pellets in the PVH of hypothyroid rats110 (Fig. 7-11A and B) reduced the concentration of TRH mRNA and TRH prohormone in TRH-secreting cells. Thyroid hormone also suppressed TRH secretion into hypophyseal-portal blood in sheep. T4 in the blood gains access to TRH-secreting neurons in the hypothalamus by way of the CSF. The hormone is taken up by epithelial cells of the choroid plexus of the lateral ventricle of the brain, bound within the cell to locally produced transthyretin (T4-binding prealbumin), and then secreted across the blood-brain barrier.111 Within the brain, T4 is converted to T3 by type II deiodinase, and T3 interacts with subtypes of the thyroid hormone receptor (TRα1, TRβ1, and TRβ2) in the PVH and other brain cells

122    Neuroendocrinology

Glucocorticoid receptors Thyroid hormone receptors Leptin receptors

Paraventricular nucleus

TRH neuron CRH

Brain stem catecholaminergic inputs Temperature

Energy state

NPY/ AGRP neurons

Arcuate nucleus

Hypothalamus tanycytes T3 T4

POMC/ CART neurons

TRH

Somatostatin

Pituitary T3 T4

TSH T4

T3 Thyroid Figure 7-9 Regulation of the hypothalamic-pituitary-thyroid axis. AGRP, agouti-related peptide; CART, cocaine- and amphetamine-regulated transcript; CRH, corticotropin-releasing hormone; NPY, neuropeptide Y; POMC, pro-opiomelanocortin; T3, triiodothyronine; T4, thyroxine; TRH, thyrotropin-releasing hormone; TSH, thyrotropin.

(see Chapter 11). In this way, the set point of the pituitarythyroid axis is determined by thyroid hormone levels within the brain.112 T3 in the circulation is not transported into brain in the same manner but presumably gains access to the paraventricular TRH neurons across the blood-brain barrier. The brain T4 transport and deiodinase system accounts for the fact that higher blood levels of T3 are required to suppress pituitary-thyroid function after administration of T3 than after administration of T4.112,113 Transthyretin is present in the brain of early reptiles and in addition is synthesized by the liver in warm-blooded animals.111 During embryogenesis in mammals, transthyretin is first detected when the blood-brain barrier appears, ensuring thyroid hormone access to the developing nervous system.

Neural Control The hypothalamus determines the set point of feedback control around which the usual feedback regulatory

responses are elicited. Lesions of the thyrotropic area lower basal thyroid hormone levels and make the pituitary more sensitive to inhibition by thyroid hormone, and high doses of TRH raise the levels of TSH and thyroid hormone. Synthesis of TRH in the PVH is regulated by feedback actions of thyroid hormones.112 The hypothalamus can override normal feedback control through an open-loop mechanism involving neuronal inputs to the hypophyseotropic TRH neurons (see Fig. 7-9). For example, cold exposure causes a sharp increase in TSH release in animals and in human newborns. Circadian changes in TSH secretion are another example of brain-directed changes in the set point of feedback control, but if thyroid hormone levels are sufficiently elevated, as in hyperthyroidism, TRH cannot overcome the inhibition. Hypothalamic regulation of TSH secretion is also influenced by two inhibitory factors, somatostatin and dopamine. Anti-somatostatin antibodies increase basal TSH levels and potentiate the response to stimuli that normally

Neuroendocrinology    123

A

B

200 180 Standard error

160 mU TSH per 100 mL

Plasma TSH (µg/mL)

50 40 30 Normal range

20 10

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Normal rats

60 40 20 0

0 0

2

4

6

8

10

PBI µg%

0

1

2

3

4

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Figure 7-10 Relationship between plasma thyrotropin (TSH) levels and thyroid hormone as determined by measurements of plasma protein-bound iodine (PBI) in humans and rats. These curves illustrate that plasma TSH levels are a curvilinear function of the plasma thyroid hormone level. A, Human studies were carried out by giving myxedematous patients successive increments of thyroxine (T4) at approximately 10-day intervals. Each point represents simultaneous measurements of plasma PBI and plasma TSH at various times in the six patients studied. B, The rat studies were performed by treating thyroidectomized animals with various doses of T4 for 2 weeks before assay of plasma TSH and plasma PBI. These curves illustrate that the secretion of TSH is regulated over the entire range of thyroid hormone levels. At the normal set point for T4, small changes above and below the control level are followed by appropriate increases or decreases in plasma TSH. (A from Reichlin S, Utiger RD. Regulation of the pituitary thyroid axis in man: relationship of TSH concentration to concentration of free and total thyroxine in plasma. J Clin Endocrinol Metab. 1967;27:251-255, copyright by The Endocrine Society. B from Reichlin S, Martin JB, Boshans RL, et al. Measurement of TSH in plasma and pituitary of the rat by a radioimmunoassay utilizing bovine TSH: effect of thyroidectomy or thyroxine administration on plasma TSH levels. Endocrinology. 1970;87:1022-1031, copyright by The Endocrine Society.)

induce TSH release in the rat, such as cold exposure or TRH administration.114 Thyroid hormone in turn inhibits the release of somatostatin, implying coordinated, reciprocal regulation of TRH and somatostatin by thyroid hormone. GH stimulates hypothalamic somatostatin synthesis and can inhibit TSH secretion. However, the physiologic role of somatostatin in the regulation of TSH secretion in humans is uncertain. Dopamine has modest effects on TSH secretion, and blockade of dopamine receptors (in the human) stimulates TSH secretion slightly. Changes in the metabolism of thyroid hormone also influence T3 homeostasis within the brain. In states of thyroid hormone deficiency, brain T3 levels are maintained by an increase in the deiodinase that converts T4 to T3.43 The pineal gland has been reported to inhibit thyroid function in some but not all studies. The pineal gland contains TRH, and in the frog its content changes with the season and with light and dark cycles independently of hypothalamic TRH.

Circadian Rhythm Plasma TSH in humans is characterized by a circadian periodicity, with a maximum between 9 p.m. and 5 a.m. and a minimum between 4 p.m. and 7 p.m. (see Fig. 7-7E).115 Smaller ultradian TSH peaks occur every 90 to 180 minutes, probably because of bursts of TRH released from the hypothalamus, and are physiologically important in controlling the synthesis and glycosylation of TSH. Glycosylation is a determinant of TSH potency.116

Temperature External cold exposure activates and high ambient temperature inhibits the pituitary-thyroid axis in animals, and analogous changes occur in humans under certain conditions.117 Exposure of infants to cold at the time of delivery causes an increase in blood TSH levels, possibly because of

alterations in the turnover and degradation of the thyroid hormones. Blood thyroid hormone levels are higher in winter than in summer among people living in cold climates but not in other climates. However, it is difficult to show that changes in environmental or body temperature in adults influence TSH secretion. For example, exposure to cold ambient temperature or central hypothalamic cooling does not modify TSH levels in young men. Behavioral changes, activation of the sympathetic nervous system, and shivering appear to be more important than the thyroid response for temperature regulation in adults. The autonomic nervous system and the thyroid axis work together to maintain temperature homeostasis in mammals, and TRH plays a role in both pathways.117 Hypothalamic TRH release is rapidly increased (30 to 45 minutes) in rats exposed to cold. Rapid inhibition of somatostatin release in the median eminence has also been documented, and both changes appear to play important roles in the rise in plasma TSH induced by cold exposure. TRH mRNA is elevated within 1 hour of cold exposure (see Fig. 7-11C and D). The regulation of hypophyseotropic TRH release and expression by cold is largely mediated by catecholamines. Noradrenergic and adrenergic fibers, originating in the brain stem, are found in close proximity to TRH nerve endings in the median eminence, and a rapid rise in TRH release was seen after norepinephrine treatment of hypothalamic fragments containing mainly median eminence.117 Brain stem adrenergic and noradrenergic fibers also make synaptic contacts with TRH neurons in the PVH (see Fig. 7-9),118 so catecholamines are likely to be involved in the regulation of TRH gene expression by cold. TRH neurons in the PVH are densely innervated by NPY terminals,119 and a portion of the NPY terminals arising from the C1, C2, C3, and A1 cell groups of the brain stem and projecting to the PVH are known to be catecholaminergic. Somatostatin, dopamine, and serotonin also play a variety of roles in the regulation of TRH.

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Figure 7-11 Direct effects of triiodothyronine (T3) on thyrotropin-releasing hormone (TRH) synthesis in the rat hypothalamic paraventricular nucleus (parvicellular division) were shown by immunohistochemical detection of pre-proTRH(25-50) after implantation of a pellet of either inactive diiodotyrosine (T2) as a control (panel A) or T3 (panel B). The T2 pellet had no effect on the concentration of pre-proTRH, whereas the TRH prohormone concentration was markedly reduced by T3 (black arrow indicates the unilateral pellet implantation). These studies demonstrated that thyroid hormone regulates the hypothalamic component of the pituitary-thyroid axis as well as the pituitary thyrotrope itself. C and D, Effects on TRH messenger ribonucleic acid (mRNA) levels detected by in situ hybridization before (panel C) and after (panel D) 1-hour exposure of a rat to 4° C cold. E to G, Effects on TRH mRNA levels of starvation (F) and leptin replacement during starvation (G) compared to control levels (E). White arrows show the location of the paraventricular nucleus. III, third ventricle; LH, lateral hypothalamus. (Photomicrographs in panels A, B, E, F, and G courtesy of Dr. R. M. Lechan. From Dyess EM, Segerson TP, Liposits Z, et al. Triiodothyronine exerts direct cell-specific regulation of thyrotropin-releasing hormone gene expression in the hypothalamic paraventricular nucleus. Endocrinology. 1988;123:2291-2297, copyright by The Endocrine Society; photomicrographs in panels C and D courtesy of Dr. P. Joseph-Bravo.)

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Stress Stress is another determinant of TSH secretion.109 In humans, physical stress inhibits TSH release, as indicated by the finding that low levels of T3 and T4 in patients with the euthyroid sick syndrome do not cause compensatory increases in TSH secretion as would occur in normal individuals.120 A number of observations demonstrate interactions between the thyroid and adrenal axes. Physiologically, the bulk of evidence suggests that glucocorticoids in humans and rodents act to blunt the thyroid axis through actions in the CNS.121 Some actions may be direct, because the TRH gene (see Fig. 7-8) contains a glucocorticoid response element consensus half-site98 and hypophyseotropic TRH neurons appear to contain glucocorticoid receptors.122 The diurnal rhythm of cortisol is opposite that of TSH (see Fig. 7-7), and acute administration of glucocorticoids can block the nocturnal rise in TSH; however, disruption of cortisol synthesis with metyrapone only modestly affects the TSH circadian rhythm. Nevertheless, several lines of evidence identify conditions in which elevated glucocorticoids are associated with stimulation of the thyroid axis. Human depression is often associated with hypercortisolism and hyperthyroxinemia, and TRH mRNA levels are elevated by glucocorticoids in a number of cell lines as well as in cultured fetal hypothalamic TRH neurons from the rat. Therefore, although glucocorticoids probably stimulate TRH production in TRH neurons, their overall inhibitory effect on the thyroid axis results from indirect glucocorticoid negative feedback on structures such as the hippocampus. Disruption of hippocampal suppression of the hypothalamic-pituitary-adrenal (HPA) axis is proposed to be involved in the hypercortisolemia commonly seen in affective illness, and disruption of hippocampal inputs to the hypothalamus has been shown to produce a rise in hypophyseotropic TRH in the rat.123

Starvation The thyroid axis is depressed during starvation, presumably to help conserve energy by depressing metabolism (Fig. 7-11E to G). In humans, T3, T4, and TSH are reduced during starvation or fasting.124 There are also changes in the thyroid axis in anorexia nervosa, including low blood levels of T3 and low-normal levels of T4 (see Chapter 11). Inappropriately low levels of TSH are found, suggesting defective activation of TRH production by low thyroid hormone levels. During starvation in rodents, reduced TRH release into hypophyseal portal blood and reduced pro-TRH mRNA levels are seen, despite lowered thyroid hormone levels.125 Reduced basal TSH levels are also usually present. The hypothyroidism seen in fasting or in the leptindeficient ob/ob mouse can be reversed by administration of leptin,126 and evidence suggests that the mechanism involves leptin’s ability to upregulate TRH gene expression in the PVH (see Fig. 7-11E to G).127 Leptin appears to act both directly through leptin receptors on hypophyseotropic TRH neurons and indirectly through its actions on other hypothalamic cell groups, such as arcuate nucleus POMC and NPY/agouti-related peptide (AgRP) neurons.128,129 TRH neurons in the PVH receive dense NPY/AgRP and POMC projections from the arcuate and express NPY and melanocortin-4 receptors (MC4R),130 and α-MSH administration partially prevents the fasting-induced drop in thyroid hormone levels.128,129 Indeed, the TRH promoter contains a signal transducer and activator of transcription (STAT) response element and a CRE that have been

demonstrated to mediate induction of TRH gene expression by leptin and α-MSH, respectively, in a heterologous cell system (see Fig. 7-8).130 The regulation of TRH by metabolic state is likely to be under redundant control, because leptin-deficient children, unlike rodents, are euthyroid,131 whereas both rodents and humans with MC4R deficiency are euthyroid.132

Infection and Inflammation The molecular basis of infection- or inflammation-induced TSH suppression is partially established. TSH secretion is inhibited by sterile abscesses; by the injection of interleukin1β (IL-1β), an endogenous pyrogen and secretory peptide of activated lymphocytes133; or by tumor necrosis factor-α (TNF-α). IL-1β stimulates the secretion of somatostatin.134 TNF-α inhibits TSH secretion directly and induces functional changes in the rat characteristic of the “sick euthyroid” state.135 It is likely that the TSH inhibition in animal models of the sick euthyroid syndrome results from cytokine-induced changes in hypothalamic and pituitary function.136 IL-6, IL-1, and TNF-α contribute to the suppression of TSH in the sick euthyroid syndrome.137

Corticotropin-Releasing Hormone Chemistry and Evolution The HPA axis is the humoral component of an integrated neural and endocrine system that functions to respond to internal and external challenges to homeostasis (stressors). The system comprises the neuronal pathways linked to release of catecholamines from the adrenal medulla (fightor-flight response) and the hypothalamic-pituitary control of ACTH release through control of glucocorticoid production by the adrenal cortex. Pituitary ACTH release is stimulated primarily by CRH and to a lesser extent by AVP (see Chapter 8). The hypophyseotropic CRH neurons are located in the parvicellular division of the PVH and project to the median eminence (see Figs. 7-3 and 7-4). In a broader context, the CRH system in the CNS is also vitally important in the behavioral response to stress. This complex system includes not only nonhypophyseotropic CRH neurons but also three CRH-like peptides (urocortin, urocortin 2 or stresscopin-related peptide, and urocortin 3 or stresscopin), at least two cognate receptors (CRH-R1 and CRH-R2), and a high-affinity CRH-binding protein, each with distinct and complex distributions in the CNS. The Schally and Guillemin laboratories demonstrated in 1955 that extracts from the hypothalamus stimulated ACTH release from the pituitary. The primary active principle, CRH, was purified and characterized from sheep in 1981 by Vale and colleagues.137a Human CRH is an amidated 41-amino-acid peptide that is cleaved from the C-terminus of a 196-amino-acid pre-prohormone precursor by PC1 and PC2 (Fig. 7-12).138 CRH is highly conserved phylogenetically; the human peptide is identical in sequence to the mouse and rat peptides but differs at seven residues from the ovine sequence. Mammalian CRH, the three urocortin peptides, fish urotensin, anuran sauvagine, and the insect diuretic peptides are members of an ancient family of peptides that evolved from an ancestral precursor early in the evolution of metazoans, approximately 500 million years ago.139 Comparison of peptide sequences in vertebrates suggests a grouping of the peptides into two subfamilies, CRHurotensin-urocortin-sauvagine and urocortin 2-urocortin 3 (Fig. 7-13).140 Urocortin and sauvagine appear to represent tetrapod orthologues of fish urotensin. Sauvagine, isolated originally from Phyllomedusa sauvagei, is an osmoregulatory

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Ser - Glu - Glu - Pro - Pro - Ile - Ser - Leu - Asp - Leu - Thr - Phe - His - Leu - Leu Arg - Glu - Val - Leu - Glu - Met - Ala - Arg - Ala - Glu - Gln - Leu - Ala - Gln - Gln Ala - His - Ser - Asn - Arg - Lys - Leu - Met - Glu - Ile - Ile - NH2 Figure 7-12  Structure of the human corticotropin-releasing hormone (CRH) gene, complementary DNA, and peptide. The sequence encoding CRH occurs at the carboxy-terminus of the prohormone. Dibasic amino acid cleavage sites (RR) and the penultimate Gly and terminal Lys (GK) are shown. AP-1, activator protein-1 binding sequence; CRE, cyclic adenosine monophosphate (cAMP)-response element; ERE, estrogen response element; GRE, glucocorticoid response element; PAM, peptidylglycine α-amidating monooxygenase; PC1/PC2, prohormone convertases 1 and 2; TATA, Goldstein-Hogness box involved in binding RNA polymerase; UTR, untranslated. (Redrawn from data of Shibahara S, Morimoto Y, Furutani Y, et al. Isolation and sequence analysis of the human corticotropinreleasing factor precursor gene. EMBO J. 1983;2:775-779.)

peptide produced in the skin of certain frogs; urotensin is an osmoregulatory peptide produced in the caudal neurosecretory system of the fish. Whereas isolation of CRH required 250,000 ovine hypothalami, the “virtual” cloning of urocortin 2 and 3 was accomplished by computer search of the human genome database.140 The CRH peptides signal by binding to CRH-R1141,142 and CRH-R2143 receptors that couple to the stimulatory G protein (Gs) and activate adenylyl cyclase. Two splice

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variants of the CRH-R2 receptor that differ in their extracellular N-terminal domain, termed CRH-R2α and CRH-R2β, have been found in both rodents and humans,144 and a third N-terminal splice variant, CRH-R2γ, has been reported in the human.145 CRH, urotensin, and sauvagine are potent agonists of CRH-R1; urocortin is a potent agonist of both receptors; and urocortins 2 and 3 are specific agonists of CRH-R2. CRH-activation of the HPA axis is mediated exclusively

CRH SEEPPISLDLTFHLLREVLEMARAEQLAQQAHSNRKLMEII urocortin DNPSLSIDLTFHLLRTLLELARTQSQRERAEQNRIIFDSV urocortin II(SRP) HPGSRIVLSLDVPIGLLQILLEQARARAAREQATTNARILARVGHC urocortin III(SCP) TKFTLSLDVPTNIMNLLFNIAKAKNLRAQAAANAHLMAQIGRRK sauvagine QGPPISIDLSLELLRKMIEIEKQEKEKQQAANNRLLLDTI urotensin-I NDDPPISIDLTFHLLRNMIEMARNENQREQAGLNRKYLDEV

Figure 7-13  Sequence comparison of members of the corticotropin-releasing hormone (CRH) peptide family. Identical or highly conserved amino acids are indicated in bold letters. SCP, stresscopin; SRP, stresscopin-related peptide.

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through CRH-R1 expressed in the corticotroph. CRH neurons projecting to the median eminence are found mostly in the PVH, although most hypothalamic nuclei contain some of these neurons (Fig. 7-14A). Some CRH fibers in the PVH also project to the brain stem, and nonhypophyseotropic CRH neurons are abundant elsewhere, primarily in limbic structures involved in processing sensory information and in regulating the autonomic nervous system. Sites include the prefrontal, insular, and cingulate cortices; amygdala; substantia nigra; periaqueductal gray; locus ceruleus; nucleus of the solitary tract; and parabrachial nucleus. In the periphery, CRH is found in human placenta, where it is upregulated 6- to 40-fold during the third trimester, in lymphocytes, in autonomic nerves, and in the gastrointestinal tract. Urocortin is expressed at highest levels in the non-preganglionic Edinger-Westphal nucleus, the lateral superior olive, and the SON nucleus of the rodent brain, with additional sites including the substantia nigra, ventral tegmental area, and dorsal raphe (see Fig. 7-14B). In the human, urocortin is widely distributed, with highest levels in the frontal cortex, temporal cortex, and hypothalamus,146 and has also been reported in the non-preganglionic Edinger-Westphal nucleus.22 In the periphery, urocortin is seen in placenta, mucosal inflammatory cells of the gastrointestinal tract, lymphocytes, and cardiomyocytes. Urocortin 2 is expressed in hypothalamic neuroendocrine and stress-related cell groups in the mouse, including the locus ceruleus, whereas urocortin 3 is expressed in hypothalamus and amygdala, and particularly in pancreatic islet beta cells.147,148 In addition to its expression in pituitary corticotrophs, CRH-R1 is found in the neocortex and cerebellar cortex, subcortical limbic structures, and amygdala, with little to no expression in the hypothalamus (see Fig. 7-14C). CRH-R1 is also found in a variety of peripheral sites in humans, including ovary, endometrium, and skin. CRHR2α is found mainly in the brain in rodents, with high levels of expression in the VMH and lateral septum (see Fig. 7-14C)149; CRH-R2β is found centrally in cerebral arterioles and peripherally in gastrointestinal tract, heart, and muscle.143,150 In humans, CRH-R2α is expressed in brain and periphery, whereas the β and γ subtypes are primarily central.144,145 Little CRH-R2 message is seen in pituitary. Although CRH-R1 appears to be exclusively involved in regulation of pituitary ACTH synthesis and release, both receptors are expressed in the rodent adrenal cortex. Data suggest that this intra-adrenal CRH-ACTH system may be involved in fine-tuning of adrenocortical corticosterone release. The CRH system is also regulated in both brain and periphery by a 37-kd high-affinity CRH-binding protein.151-153 This factor was initially postulated from the observation that CRH levels rise dramatically during the second and third trimesters of pregnancy without activating the pituitary-adrenal axis. Among hypophyseotropic factors, CRH is the only one for which a specific binding protein (in addition to the receptor) exists in tissue or blood. The placenta is the principal source of pregnancyrelated CRH-binding protein. Human and rat CRH-binding proteins are homologous (85% amino acid identity), but in the rat the protein is expressed only in brain. The binding protein is species specific; bovine CRH, which is almost identical in sequence to rat and human CRH, has a lower affinity of binding to the human binding protein. The functional significance of the CRH-binding protein is not fully understood. CRH-binding protein does not bind to the CRH receptor but does inhibit CRH action. For this reason, CRH-binding protein probably acts to

modulate CRH actions at the cellular level. Corticotroph cells in the anterior pituitary have membrane CRH receptors and intracellular CRH-binding protein; conceivably, the binding protein acts to sequester or terminate the action of membrane-bound CRH. CRH-binding protein is present in many regions of the CNS, including cells that synthesize CRH and cells that receive innervation from CRH-containing neurons. The anatomic distribution of the protein, the variability of its location in relation to the presence of CRH, and its relative sparseness in the CRH tuberohypophyseal neuronal system suggest a control system that is as yet poorly understood. Transgenic mouse models with both overexpression and gene deletion of the CRH-binding protein have been produced with little effect on basal or stress activation of the HPA axis (reviewed by Bale and Vale154). Structure-activity relationship studies have demonstrated that both C-terminal amidation and an α-helical secondary structure are important for biologic activity of CRH. The first CRH antagonist described was termed α-helical CRH(9-41).155 A second, more potent antagonist, termed astressin, has the structure cyclo(30-33)(D-Phe12, Nle,12 Glu,12 Lys12)hCRH(12-41).156 Both peptides are somewhat nonspecific, antagonizing both CRH-R1 and CRH-R2. Because of the anxiogenic activity of CRH and urocortin, a number of pharmaceutical companies have developed small-molecule CRH antagonists; several of these are currently in clinical trials for anxiety and depression (see later discussion). Thus far, this structurally diverse group of small-molecule compounds, including antalarmin, CP-154,526, and NBI27914, are potent antagonists of CRH-R1, with little activity at CRH-R2. The efficacy of these compounds across the entire behavioral, neuroendocrine, and autonomic repertoire of response to stress has been demonstrated in a number of laboratory animal studies. For example, oral administration of antalarmin in a social stress model in the primate (introduction of strange males) reduced behavioral measures of anxiety such as lack of exploratory behavior, decreased plasma ACTH and cortisol, and reduced plasma epinephrine and norepinephrine.157 Other preclinical studies in rhesus monkeys have compared the pharmacologic profiles of astressin B and antalarmin.158 A peptide antagonist with 100-fold selectivity for the CRH-R2β receptor, (D-Phe11, His12)sauvagine 11-40 or anti-sauvagine-30, has also been described.159

Effects on the Pituitary and Mechanism of Action Administration of CRH to humans causes prompt release of ACTH into the blood, followed by secretion of cortisol (Fig. 7-15) and other adrenal steroids including aldosterone. Most studies have used ovine CRH, which is more potent and longer acting than human CRH, but human and porcine CRHs appear to have equal diagnostic value. The effect of CRH is specific to ACTH release and is inhibited by glucocorticoids. As mentioned earlier, CRH acts on the pituitary corticotroph primarily by binding to CRH-R1 and activating adenylyl cyclase. The concentration of cAMP in the tissue is increased in parallel with the biologic effects and is reduced by glucocorticoids. The rate of transcription of the mRNA that encodes the ACTH prohormone POMC is also enhanced by CRH.

Extrapituitary Functions CRH and the urocortin peptides have a wide range of biologic activities in addition to the hypophyseotropic role of CRH in regulating ACTH synthesis and release. Centrally, these peptides have behavioral activities in anxiety, mood,

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Figure 7-14 Distribution of messenger RNA sequences for corticotropin-releasing hormone (CRH) (A), urocortin (B), and the CRH receptors CRH-R1 (C, circles) and CRH-R2 (C, triangles) in the rat brain. A1, Noradrenergic cell group 1; A5, noradrenergic cell group 5; ac, anterior commissure; AMB, nucleus ambiguus; Amyg, amygdala; AON, anterior olfactory nucleus; APit, anterior pituitary; AP, area postrema; BLA, basolateral amygdala; BNST, bed nucleus of the stria terminalis; CA1-4, Fields CA1-4 of Ammon’s horn; cc, corpus callosum; CeA, central nucleus amygdala; CBL, cerebellum; CG, central gray; CingCx, cingulate cortex; CoA, cortical nucleus amygdala; DBB, nucleus of the diagonal band; DG, dentate gyrus; Dors col, dorsal column nuclei; DR, dorsal raphe; DVC, dorsal vagal complex; EP/CLa, endopiriform nucleus/clawtrum; EW, Edinger-Westphal nucleus, non-cholinergic; FrCx, frontal cortex; GP, globus pallidus; HIP, hippocampus; HYP, hypothalamus; IC, inferior colliculus; IGL, intergeniculate leaflet; IO, inferior olivary complex; IP interpeduncular nucleus; LC, locus coeruleus; LDTg, laterodorsal tegmental nucleus; LHA, lateral hypothalamic area; LRN, lateral reticular nucleus; LS, lateral septum; LSO, lateral superior olivary nucleus; MA, medial amygdala; MB, mammillary body; ME, median eminence; mfb, medial forebrain bundle; MoV, motor nucleus of the trigeminal nerve; MS, medial septum; Mid Thal, midline thalamic nuclei; mfb, medial forebrain bundle; MPO, medial preoptic area; MR, medial raphe; MVN, medial vestibular nucleus; NTS, nucleus of the tractus solitarius; OB, olfactory bulb; OccCx, occipital cortex; OVLT, organum vasculosum of the lamina terminalis; PAG, periaqueductal gray; ParCx, parietal cortex; PB, parabrachial nucleus; PHA, posterior hypothalamic area; PoA, preoptic hypothalamic area; POR, perioculomotor nucleus; PP, posterior pituitary; PPTg, peripeduncular tegmental nucleus; Pretect, pretectal region; PVH, paraventricular nucleus of the hypothalamus; R, raphe; RN, red nucleus; SC, superior colliculus; SCN, suprachiasmatic nucleas; SI, substantia innominata; SN, substantia nigra; SNV, spinal trigeminal nucleus; SO, supraoptic nucleus; Sp cord, spinal cord; st, stria terminalis; SUM, supramammillary nucleus; V/Vest, vestibular nuclei; VII, facial nucleus; VMH, ventral medial nucleus of the hypothalamus; XII, hypoglossal nucleus. (From Swanson LW, Sawchenko PE, Rivier J, et al. Organization of ovine corticotropin-releasing factor immunoreactive cells and fibers in the rat brain: an immunohistochemical study. Neuroendocrinology. 1983;36:165-186; Bittencourt JC, Vaughan J, Arias C, et al. Urocortin expression in rat brain: evidence against a pervasive relationship of urocortin-containing projections with targets bearing type 2 CRF receptors. J Comp Neurol. 1999;415:285-312, Fig. 17; Steckler T, Holsboer F. Corticotropin-releasing hormone receptor subtypes and emotion. Biol Psychol. 1999;46:1480-1508, Fig. 1.)

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Figure 7-15 Comparison of plasma immunoreactive adrenocorticotropic hormone (IR-ACTH) (A) and plasma cortisol (B) responses to ovine corticotropin-releasing hormone in control subjects, patients with depression, and patients with Cushing’s disease. (From Gold PW, Loriaux DL, Roy A, et al. Responses to corticotropin-releasing hormone in the hypercortisolism of depression and Cushing’s disease: pathophysiologic and diagnostic implications. N Engl J Med. 1986;314:1329-1335.)

arousal, locomotion, reward, and feeding160,161 and increase sympathetic activation. Many of the nonhypophyseotropic behavioral and autonomic functions of these peptides can be viewed as complementary to activation of the HPA axis in the maintenance of homeostasis under exposure to stress. In the periphery, activities have been reported in immunity, cardiac function, gastrointestinal function, and reproduction.162 The CRH and urocortin peptides have a repertoire of behavioral and autonomic actions after central admini­ stration that suggests a role for these pathways in medi­ ating the behavioral-autonomic components of the stress response. Hyperactivity of the HPA axis is a common neuroendocrine finding in affective disorders (see Fig. 7-15).160,163 Normalization of HPA regulation is highly predictive of successful treatment. Defective dexamethasone

suppression of CRH release, implying defective cortico­ steroid receptor signaling, is seen not only in depressed patients but also in healthy subjects with a family history of depression.164 Depressed patients also show elevated levels of CRH in the CSF.165 Extensive behavioral testing in a variety of mutant mouse models with genetically altered expression of the CRH ligands or receptors has generally supported the hypothesis that activation of central CRH pathways is a critical neurobiologic substrate of anxiety and depressive states.154,161 Central administration of CRH or urocortin activates neuronal cell groups involved in cardiovascular control and increases blood pressure, heart rate, and cardiac output.166 However, urocortin is expressed in cardiac myocytes, and intravenous administration of CRH or urocortin decreases blood pressure and increases heart rate in most species, including humans.166 This hypotensive effect is probably mediated peripherally, because ganglion blockade did not disrupt the hypotensive effects of intravenous urocortin. Furthermore, high levels of CRH-R2β have been seen in the cardiac atria and ventricles,143,150 and knockout of the CRH-R2 gene in the mouse eliminated the hypotensive effects of intravenous urocortin administration.167,168 Cytokines have an important role in extinguishing inflammatory responses through activation of CRH and AVP neurons in the PVH and subsequent elevation of antiinflammatory glucocorticoids. Interestingly, CRH is generally proinflammatory in the periphery, where it is found in sympathetic efferents, in sensory afferent nerves, in leukocytes, and in macrophages in some species.162,169 CRH also functions as a paracrine factor in the endometrium, where it may play a role in decidualization and implantation and may act as a uterine vasodilator.162 The relative contributions of each of the CRH-urocortin peptides and receptors to the various biologic functions reported has been the topic of considerable analysis, given the receptor-specific antagonists already described and the availability of CRH, CRH-R1, and CRH-R2 knockout mice for study (reviewed by Bale and Vale154 and Keck and colleagues161). Examination of three potent stressors—restraint, ether, and fasting—demonstrated that other ACTH secretagogues, such as AVP, oxytocin, and catecholamines, could not replace CRH in its role in mounting the stress response. In contrast, augmentation of glucocorticoid secretion by a stressor after prolonged stress was not defective in CRH knockout mice, implicating CRH-independent mechanisms. Although CRH is a potent anxiogenic peptide, CRH knockout mice exhibit normal anxiety behaviors in, for example, conditioned fear paradigms. The nonpeptide CRH-R1–specific antagonist CP-154,526 was anxiolytic in a shock-induced freezing paradigm in both wild-type and CRH knockout mice, suggesting that the anxiogenic activity is a CRH-like peptide acting at the CRH-R1 receptor (see review154). CRH and urocortin peptides also have potent anorexigenic activity, implicating the CRH system in stress-induced inhibition of feeding. However, in CRH knockout mice, stress-induced inhibition of feeding and suppression of the proestrous LH surge by restraint remained intact. Both CRH-R1 and CRH-R2 knockout strains had normal weight and feeding behaviors but were distinctly different from wild-type mice in the anorexigenic response to centrally administered urocortin or CRH. The CRH-R1–deficient mice lacked the acute anorexigenic response (0 to 1.5 hours) to urocortin seen in wild-type mice. Wild-type and CRH-R1 knockout mice exhibited comparable reductions in feeding 3 to 11 hours after administration. In contrast,

130    Neuroendocrinology the late phase of urocortin responsiveness appeared to depend on the presence of CRH-R2. Therefore, signaling through CRH-R1 and CRH-R2 plays a complex role in the acute effects of stress on feeding behavior (see review154). Additional gene knockout studies have suggested that urocortin 2 plays a physiologic role in female mice to dampen basal daily rhythms of the HPA axis and reduce behavioral coping mechanisms in response to chronic stress.147 Urocortin 3 may have a primary action to augment insulin secretion in response to the metabolic stress of excessive calorie intake.148

Clinical Applications No approved therapeutic application of CRH or CRH-like peptides exists, although the peptide has been demonstrated to have a number of activities in human and primate studies. Intravenous administration was found to stimulate energy expenditure, but CRH is an unlikely pharmaceutical target for inducing weight loss. The development of small-molecule, orally available, CRH-R1 antagonists has produced considerable interest in their potential for treatment of anxiety and depression.165,170 In particular, the compound R121919 was studied in phase 1 and 2a clinical trials before its discontinuance. These studies of 20 patients demonstrated significant reductions in anxiety and depression scores, using ratings determined by patient or clinician, and also demonstrated the compound’s safety and favorable side-effect profile, including a lack of effect on endocrine function or body weight gain.171

Feedback Control The administration of glucocorticoids inhibits ACTH secretion, and, conversely, removal of the adrenals (or administration of drugs that impair secretion of glucocorticoids) leads to increased ACTH release. The set point of pituitary feedback is determined by the hypothalamus acting through hypothalamic releasing hormones CRH and AVP (see Chapter 8).172-175 Glucocorticoids act on both the pituitary corticotrophs and the hypothalamic neurons that secrete CRH and AVP. These regulatory actions are analogous to the control of the pituitary-thyroid axis. However, whereas TSH becomes completely unresponsive to TRH when thyroid hormone levels are sufficiently high, severe neurogenic stress and large amounts of CRH can break through the feedback inhibition due to glucocorticoids. A still higher level of feedback control is exerted by glucocorticoid-responsive neurons in the hippocampus that project to the hypothalamus; these neurons affect the activity of CRH hypophyseotropic neurons and determine the set point of pituitary responsiveness to glucocorticoids.175 A comprehensive review of glucocorticoid effects on CRH and AVP and regulation of the HPA axis emphasized the complexity of this control beyond that of a simple closed-loop feedback.176 Glucocorticoids are lipid soluble and freely enter the brain through the blood-brain barrier.174 In brain and pituitary, they can bind to two receptors. Type 1 (encoded by NR3C2) is called the mineralocorticoid receptor because it binds aldosterone and glucocorticoids with high affinity). Type 2 (NR3C1), the glucocorticoid receptor, has low affinity for mineralocorticoids.173-175 Classic glucocorticoid action involves binding of the steroid-receptor complex to regulator sequences in the genome. MR are saturated by basal levels of glucocorticoids, whereas GR are not saturated under basal conditions but approach saturation during peak phases of the circadian rhythm and during stress. These differences and differences in regional

distribution within the brain suggest that MR determine basal activity of the hypothalamic-pituitary axis and GR mediate stress responses. In the pituitary, glucocorticoids inhibit secretion of ACTH and the synthesis of POMC mRNA; in the hypothalamus, they inhibit the secretion of CRH and AVP and the synthesis of their respective mRNAs, although with distinct temporal patterns.174-176 Neuron membrane excitability and ion transport properties are suppressed by changes in glucocorticoid-directed synthesis of intra­ cellular protein. Glucocorticoids can exert additional rapid signaling events in neurons, including an endocannabinoid-mediated suppression of synaptic excitation.177 These rapid events involve membrane-associated complexes and are independent of changes in gene transcription or acute protein translation, but the exact mechanisms and nature of the receptors are still under investigation.178 Glucocorticoids block stress-induced ACTH release. The latency of the inhibitory effect is so short (1 hour) clearly acts through genomic mechanisms. Glucocorticoid receptors are also found outside the hypothalamus, in the septum and in the amygdala,174,175 and these structures are involved in the psychobehavioral changes of hypercortisolism and hypocortisolism. In all of these areas, apart from the CRH neurons of the PVH, glucocorticoids have either a stimulatory or a neutral effect on CRH gene expression.176 Hippocampal neurons are reduced in number by prolonged elevation of glucocorticoids during chronic stress.175

Neural Control Significant physiologic or psychological stressors evoke an adaptive response that commonly includes activation of both the HPA axis and the sympathoadrenal axis. The end products of these pathways then help to mobilize resources to cope with the physiologic demands in emergency situations, acutely through the fight-or-flight response and over the long term through systemic effects of glucocorticoids on functions such as gluconeogenesis and energy mobilization (see Chapter 15). The HPA axis also has unique stress-specific homeostatic roles, the best example being the role of glucocorticoids in downregulating immune responses after infection and other events that stimulate cytokine production by the immune system. The PVH is the primary hypothalamic nucleus responsible for providing the integrated whole-animal response to stress.176,179,180 This nucleus contains within it three major types of effector neurons that are spatially distinct from one another: (1) magnicellular oxytocin and AVP neurons that project to the posterior pituitary and participate in the regulation of blood pressure, fluid homeostasis, lactation, and parturition; (2) neurons projecting to the brain stem and spinal cord that regulate a variety of autonomic responses, including sympathoadrenal activation; and (3) parvicellular CRH neurons that project to the median eminence and regulate ACTH synthesis and release. Many CRH neurons coexpress AVP, which acts as an auxiliary ACTH secretagogue, synergistic with CRH. AVP is regulated quite differently, in parvicellular versus magnicellular neurons, but also somewhat differently from CRH by stressors in parvicellular cells that express both peptides.176 Different stressors result in different patterns of activation of the three major visceromotor cell groups within the PVH, as measured by the general neuronal activation marker Fos (Fig. 7-16). For example, salt loading

Neuroendocrinology    131

Figure 7-16 Regulation of neurons of the paraventricular nucleus (PVH) by diverse stressors. ADX, adrenalectomy; CRF, corticotropin-releasing factor in situ hybridization (dark-field); dp, dorsal parvicellular; Fos, FOS immunoreactivity (bright-field); IL-1, interleukin-1; pm, posterior magnicellular; NGFI-B, nerve growth factor 1β in situ hybridization (dark-field); mp, medial parvicellular. (From Sawchenko PE, Brown ER, Chan RK, et al. The paraventricular nucleus of the hypothalamus and the functional neuroanatomy of visceromotor responses to stress. Prog Brain Res. 1996; 107:201-222.)

downregulates CRH mRNA in parvicellular CRH cells, upregulates CRH in a small number of magnicellular CRH cells, but activates only magnicellular cells. Hemorrhage activates every division of the PVH, whereas cytokine administration primarily activates parvicellular CRH cells, with some minor activation of magnicellular and autonomic divisions. The synthesis and release of AVP, which regulates renal water absorption and vascular smooth muscle, are controlled mainly by the volume and tonicity of the blood. This information is relayed to the magnicellular AVP cell

through the nucleus of the solitary tract and A1 noradrenergic cell group of the ventrolateral medulla and through projections from a triad of CVOs lining the third ventricle: the SFO, the medial preoptic nucleus (MePO), and the OVLT. Oxytocin is primarily involved in reproductive functions, such as parturition, lactation, and milk ejection, although it is cosecreted with AVP in response to osmotic and volume challenges, and oxytocin cells receive direct projections from the nucleus of the solitary tract as well as from the SFO, MePO, and OVLT. In contrast to the neurosecretory neurons functionally defined by the three

132    Neuroendocrinology

Limbic

A

SFO

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BST MePO

NTS

OT AVP

Somatosensory

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Bloodborne OT, AVP

Bloodborne SFO Limbic

MePO BST

B

Glucocorticoids

OVLT

PVT

Visual

Nociceptive Somatosensory IGL

PVH

CG PPN, LDT

PB

Visceral

NTS

CRF PP, PIN

HYP

Auditory

C1

ACTH Figure 7-17  A, Neuronal inputs to magnicellular and, B, parvicellular neurons of the paraventricular nucleus (PVH). ACTH, adrenocorticotropic hormone; A1, A1 noradrenergic cell group; AVP, arginine vasopressin; BST, bed nucleus of the stria terminalis; C1, C1 adrenergic cell group; CG, central gray; CRF, corticotropin-releasing factor; HYP, hypothalamus; IGL, intergeniculate leaflet; LDT, laterodorsal tegmental nucleus; MePO, medial preoptic nucleus; NTS, nucleus of the tractus solitarius; OT, oxytocin; OVLT, organum vasculosum of the lamina terminalis; PB, parabrachial nucleus; PIN, posterior intralaminar nucleus; PP, peripeduncular nucleus; PPN, pedunculopontine nucleus; PVT, paraventricular nucleus of the thalamus; SFO, subfornical organ; SO, supraoptic nucleus. (From Sawchenko PE, Brown ER, Chan RK, et al. The paraventricular nucleus of the hypothalamus and the functional neuroanatomy of visceromotor responses to stress. Prog Brain Res. 1996;107:201-222.)

peptides CRH, oxytocin, and AVP, PVH neurons projecting to brain stem and spinal cord include neurons expressing each of these peptides. In the rodent, a wide variety of stressors have been determined to activate parvicellular CRH neurons, including cytokine injection, salt loading, hemorrhage, adrenalectomy, restraint, foot shock, hypoglycemia, fasting, and ether exposure. In contrast to the simplicity of inputs to magnicellular cells (Fig. 7-17A), it is not surprising that parvicellular CRH neurons receive a diverse and complex assortment of inputs (Fig. 7-18; see Fig. 7-17B). These inputs are divided into three major categories: brain stem, limbic forebrain, and hypothalamus. Because the PVH is not known to receive any direct projections from the cerebral cortex or thalamus, stressors relating to emotional or cognitive processing must involve indirect relay to the PVH. Visceral sensory input to the PVH involves primarily two pathways. The nucleus of the solitary tract, the primary recipient of sensory information from the thoracic and abdominal viscera, sends dense catecholaminergic projections to the PVH, both directly and through relays in the ventrolateral medulla. These brain stem projections account for about half of the NPY fibers present in the PVH. A second major input responsible for transducing signals from bloodborne substances derives from three CVOs adjacent to the third ventricle: the SFO, the OVLT, and the

MePO. These pathways account for activation of CRH neurons by what are referred to as systemic or physiologic stressors.180 By contrast, what are termed neurogenic, emotional, or psychological stressors involve, in addition, nociceptive or somatosensory pathways as well as cognitive and affective brain centers. Using elevation of Fos as an indicator of neuronal activation, detailed studies have compared PVHprojecting neurons activated by IL-1 treatment (systemic stressor) versus foot shock (neurogenic stressor).180 Only catecholaminergic solitary tract nucleus and ventrolateral medulla neurons were activated by moderate doses of IL-1. In contrast, foot shock activated neurons of the solitary tract nucleus and ventrolateral medulla but also cell groups in the limbic forebrain and hypothalamus. Notably, pharmacologic or mechanical disruption of the ascending catecholaminergic fibers blocked IL-1–mediated activation but not foot shock–mediated activation of the HPA axis. Data suggest that pathways activated by other neurogenic and systemic stressors may overlap significantly with those activated by foot shock and IL-1 treatment, respectively.179,180 Except for the catecholaminergic neurons of the nucleus of the solitary tract and ventrolateral medulla, parts of the bed nucleus of the stria terminalis, and the dorsomedial nucleus of the hypothalamus, many inputs to the PVH, such as those deriving from the prefrontal cortex and

Neuroendocrinology    133 Hippocampus Prefrontal cortex

Processive (Neurogenic) Stress (fear, restraint) Lateral septum

BST

Hypothalamic GABAergic inputs Paraventricular nucleus

GABA

MeA

5-HT

Hypothalamic glutamatergic inputs

Hypothalamic glutamatergic inputs

Brain stem catecholaminergic inputs NTS

CRH neuron

Subfornical org. OVLT MePO

Systemic (Physiological) Stress (osmotic challenge, macromolecules)

Glucocorticoid receptors Cytokine receptors Leptin receptors

Raphe nucleus

Prostaglandins Brain stem vasculature

Arcuate, NPY and POMC neurons

Systemic (Physiological) Stress (cytokines, hypoxia, hemorrhage)

Hypothalamus

to CNS CRH AVP

CRIF?

IL-1, IL-2, IL-6, TNFα

Pituitary Immune cells

ACTH

Cortisol

Adrenal Figure 7-18 Regulation of the hypothalamic-pituitary-adrenal axis. ACTH, adrenocorticotropic hormone; AVP, arginine vasopressin; BST, bed nucleus of the stria terminalis; CNS, central nervous system; CRH, corticotropin-releasing hormone; CRIF, corticotropin release–inhibiting factor; GABA, γ-aminobutyric acid; 5-HT, 5-hydroxytryptamine; IL, interleukin; MeA, medial amygdala; MePO, medial preoptic nucleus; NPY, neuropeptide Y; NTS, nucleus of the tractus solitarius; OVLT, organum vasculosum of the lamina terminalis; POMC, pro-opiomelanocortin; TNFα, tumor necrosis factor-α.

lateral septum, are thought to act indirectly through local hypothalamic glutamatergic181 and GABAergic neurons182 with direct synapses to the CRH neurons. The bed nucleus of the stria terminalis is the only limbic region with prominent direct projections to the PVH. With substantial projections from the amygdala, hippocampus, and septal nuclei, this region may serve as a key integrative center for transmission of limbic information to the PVH.179

Inflammation and Cytokines Stimulation of the immune system by foreign pathogens leads to a stereotyped set of responses orchestrated by the CNS. This constellation of stereotyped responses result from the complex interaction of the immune system and the CNS. They are mediated in large part by the hypothalamus and include coordinated autonomic, endocrine, and

134    Neuroendocrinology behavioral components with adapative consequences to restore homeostasis. It is now clear that cytokines produced by peripheral circulating cells of the immune system and central glial cells mediate the CNS responses. Early evidence supporting this hypothesis was provided by the seminal observations that cytokines such as IL-1β can activate the HPA axis.183-185 Neuroimmunology, the discipline arising from the study of reciprocal interactions between the neuroendocrine and immune systems, and particularly the role of cytokines in mediating cachexia, is covered fully in Chapter 35. This section focuses on cytokines and activation of the HPA axis. The resultant glucocorticoid secretion acts as a classic negative feedback to the immune system to dampen its response. In general, glucocorticoids inhibit most limbs of the immune response, including lymphocyte proliferation, production of immunoglobulins, cytokines, and cytotoxicity. These inhibitory reactions form the basis of the anti-inflammatory actions of glucocorticoids. Glucocorticoid feedback on immune responses is regulatory and beneficial because loss of this function makes animals with adrenal insufficiency vulnerable to inflammation. However, this feedback response can have pathophysiologic consequences, because chronic activation of the HPA axis can certainly be detrimental.186,187 Indeed, it is well established that chronic stress can lead to immunosuppression. The fact that products of inflammation such as IL-1β can activate the HPA axis suggests the operation of a negative feedback control loop to regulate the intensity of inflammation. The role of the hypothalamus in regulating pituitary-adrenal function is an excellent example of neuroimmunomodulation. Proposed models to explain how immune system signals might act on the CNS to modulate homeostatic circuits through integration of vagal input, peripheral cytokine interactions with receptors in the CVOs and cerebral blood vessels, and local production of cytokines within the CNS are explored in Chapter 35.

Other Factors Influencing Secretion of Corticotropin Circadian Rhythms.  Levels of ACTH and cortisol peak in the early morning, fall during the day to reach a nadir at about midnight, and begin to rise between 1 a.m. and 4 a.m. (see Fig. 7-7). Within the circadian cycle, approximately 15 to 18 pulses of ACTH can be discerned, their heights varying with the time of day.188 The set point of feedback control by glucocorticoids also varies in a circadian pattern. Pituitary-adrenal rhythms are entrained to the light-dark cycle and can be changed over several days by exposure to an altered light schedule. It has long been assumed that the rhythm of ACTH secretion is driven by CRH rhythms, and CRH knockout mice were found to exhibit no circadian rhythm in corticosterone production. Remarkably, however, a diurnal rhythm in corticosterone was restored by a constant infusion of CRH to CRH knockout mice,189 suggesting that CRH is necessary to permit pituitary or adrenal responsiveness to another diurnal rhythm generator. Corticotropin Release–Inhibiting Factor.  Disconnection of the pituitary from the hypothalamus in several species leads to increased basal levels of ACTH, and certain responses to physical stress (in contrast to psychological stress) are retained in such animals. These observations led several investigators to postulate the existence of an ACTH inhibitory factor, analogous to dopamine in the control of PRL secretion and to somatostatin in the control of GH

secretion. Candidate hypothalamic peptides to inhibit ACTH release at the level of the pituitary include somatostatin, atrial natriuretic peptide, activins and inhibins, and peptide sequence 178 to 199 of the TRH prohormone.190 There is not yet a consensus on the existence or identity of a physiologically relevant ACTH releaseinhibiting factor.

Growth Hormone–Releasing Hormone Chemistry and Evolution Evidence for neural control of GH secretion came from studies of its regulation in animals with lesions of the hypothalamus191 and from the demonstration that hypothalamic extracts stimulate the release of GH from the pituitary. After it was shown that GH is released episodically, follows a circadian rhythm, responds rapidly to stress, and is blocked by pituitary stalk section, the concept of neural control of GH secretion became a certainty. However, it was only with the discovery of the paraneoplastic syndrome of ectopic GHRH secretion by pancreatic adenomas in humans that sufficient starting material became available for peptide sequencing and subsequent cloning of a complementary deoxyribonucleic acid (cDNA).192-195 Two principal molecular forms of GHRH occur in the human hypothalamus: GHRH(1-44)-NH2 and GHRH(1-40) (Fig. 7-19).196 As with other neuropeptides, the various forms of GHRH arise from post-translational modification of a larger prohormone.192,197 The N-terminal tyrosine of GHRH (or histidine in rodent GHRH) is essential for bioactivity, but a C-terminal NH2 group is not. Fragments as short as (1-29)-NH2 are active, but GHRH(1-27)-NH2 is inactive. A circulating type IV dipeptidylpeptidase potently inactivates GHRH to its principal and more stable metabolite, GHRH(3-44)-NH2,198 which accounts for most of the immunoreactive peptide detected in plasma. As in the case of GnRH, there are GHRH differences among species; the peptides from seven species range in sequence homology with the human peptide from 93% in the pig to 67% in the rat.196 The C-terminal end of GHRH exhibits the most sequence diversity among species, consistent with the exon arrangement of the gene and dispensability of these residues for GHRH receptor binding. Despite its importance for elucidation of GHRH structure, ectopic secretion of the peptide is a rare cause of acromegaly. Fewer than 1% of acromegalic patients have elevated plasma levels of GHRH (see Chapter 9).199 Approximately 20% of pancreatic adenomas and 5% of carcinoid tumors contain immunoreactive GHRH, but most are clinically silent.200 In addition to expression in the hypothalamus, the GHRH gene is expressed eutopically in human ovary, uterus, and placenta,201 although its function in these tissues is not known. Studies in rat placenta indicate that an alternative transcriptional start site 10 kilobases upstream from the hypothalamic promoter is used, together with an alternatively spliced exon 1a.202

Growth Hormone–Releasing Hormone Receptor The GHRH receptor is a member of a subfamily of G protein–coupled receptors that includes receptors for VIP, pituitary adenylyl cyclase–activating peptide, secretin, glucagon, glucagon-like peptide 1, calcitonin, parathyroid hormone or parathyroid hormone–related peptide, and gastric inhibitory polypeptide.203,204 GHRH elevates intracellular cAMP by its receptor coupling to a stimulatory G protein (Gs), which activates adenylyl cyclase, increases

Neuroendocrinology    135 Placental Promoter

Hypothalamic Promoter

GSH-1 x 5 Alternative Exon 1

2 3

Exon 1

TATA 4

Intron

Intron

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3'UTR

Signal Peptide

GHRH

Poly A 100 bp

PC1/PC2 [GHRH (1-44)] – Gly – Lys and GHRH (1-40) CPE PAM GHRH (1-44) – NH2

Tyr – Ala – Asp – Ala – Ile – Phe – Thr – Asn – Ser – Tyr – Arg – Lys – Val – Leu – Gly – Gln – Leu – Ser – Ala – Arg – Lys – Leu – Leu – Gln – Asp – Ile – Met – Ser – Arg – Gln – Gln – Gly – Glu – Ser – Asn – Gln – Glu – Arg – Gly – Ala – Arg – Ala – Arg – Leu – NH2 Type IV dipeptidylpeptidase GHRH (3-44) – NH2

Figure 7-19 Diagram illustrating the genomic organization, messenger RNA structure, and post-translational processing of the human growth hormone– releasing hormone (GHRH) prohormone. The five GSH-1 homeodomain transcription factor binding sites in the proximal promoter have been characterized in the rat gene. All of the amino acid residues required for bioactive GHRH peptides are encoded by exon 3. An amino-terminal exopeptidase that cleaves the Tyr-Ala dipeptide is primarily responsible for inactivation of GHRH peptides in extracellular compartments. CPE, carboxypeptidase E; PAM, peptidylglycine α-amidating monooxygenase; PC1/PC2, prohormone convertases 1 and 2; TATA, Goldstein-Hogness box involved in binding RNA polymerase; UTR, untranslated region. (Compiled from data of Mayo KE, Cerelli GM, Lebo RV, et al. Gene encoding human growth hormone-releasing factor precursor: structure, sequence, and chromosomal assignment. Proc Natl Acad Sci U S A. 1985;82:63-67; Frohman LA, Downs TR, Chomczynski P, et al. Growth hormone-releasing hormone: structure, gene expression and molecular heterogeneity. Acta Paediatr Scand Suppl. 1990;367:81-86; González-Crespo S, Boronat A. Expression of the rat growth hormonereleasing hormone gene in placenta is directed by an alternative promoter. Proc Natl Acad Sci U S A. 1991;88:8749-8753; and Mutsuga N, Iwasaki Y, Morishita M, et al. Homeobox protein Gsh-1-dependent regulation of the rat GHRH gene promoter. Mol Endocrinol. 2001;15:2149-2156.)

intracellular free Ca2+, releases preformed GH, and stimulates GH mRNA transcription and new GH synthesis (see Chapter 8).205 GHRH also increases pituitary phosphatidylinositol turnover. Nonsense mutations in the human GHRH receptor gene are the cause of rare familial forms of GH deficiency206,207 and indicate that no other gene product can fully compensate for the specific receptor in pituitary.

Effects on the Pituitary and Mechanism of Action Intravenous administration of GHRH to individuals with normal pituitaries causes a prompt, dose-related increase in serum GH that peaks after 15 and 45 minutes, followed by a return to basal levels by 90 to 120 minutes (Fig. 7-20).208 A maximally stimulating dose of GHRH is approximately 1 µg/kg, but the response differs considerably among individuals and within the same individual tested on different occasions, presumably because of endogenous cosecretagogue and somatostatin tone that exists at the time of GHRH injection. Repeated bolus administration or sustained infusions of GHRH over several hours cause a modest decrease in the subsequent GH secretory response to acute GHRH administration. However, unlike the marked desensitization of the GnRH receptor and decline in

circulating gonadotropins that occur in response to continuous GnRH exposure, pulsatile GH secretion and insulin-like growth factor type 1 (IGF1) production are maintained by constant GHRH in the human.208 This response suggests the involvement of additional factors that mediate the intrinsic diurnal rhythm of GH, and these factors are addressed in the following sections. The pituitary effects of a single injection of GHRH are almost completely specific for GH secretion, and there is minimal evidence for any interaction between GHRH and the other classic hypophyseotropic releasing hormones.208 GHRH has no effect on gut peptide hormone secretion. The GH secretory response to GHRH is enhanced by estrogen administration, glucocorticoids, and starvation. Major factors known to blunt the response to GHRH in humans are somatostatin, obesity, and advancing age. In addition to its role as a GH secretagogue, GHRH is a physiologically relevant growth factor for somatotrophs. Transgenic mice expressing a GHRH cDNA coupled to a suitable promoter developed diffuse somatotroph hyperplasia and eventually pituitary macroadenomas.209,210 The intracellular signal transduction pathways mediating the mitogenic action of GHRH are not known with certainty but probably involve an elevation of adenylyl cyclase

136    Neuroendocrinology 160 GHRH + ghrelin Ghrelin GHRH

GH (µg/L)

120

80

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Time (min) Figure 7-20 Response of normal men to growth hormone–releasing hormone GHRH(1-29) (1 µg/kg), ghrelin (1 µg/kg), or the combination of GHRH(1-29) and ghrelin administered by intravenous injection. Note the prompt release of growth hormone (GH), followed by a rather prolonged fall in hormone level in response to the administered secretagogues. Ghrelin alone was more efficacious than GHRH(1-29), and there was an additive effect when the two peptides were administered simultaneously. (From Arvat E, Macario M, Di Vito L, et al. Endocrine activities of ghrelin, a natural growth hormone secretagogue (GHS), in humans: comparison and interactions with hexarelin, a nonnatural peptidyl GHS, and GH-releasing hormone. J Clin Endocrinol Metab. 2001;86:1169-1174.)

activity. Several lines of evidence support this conclusion, including the association of activating mutations of the Gsα polypeptide in many human somatotroph adenomas.211

Extrapituitary Functions GHRH has few known extrapituitary functions. The most important may be its activity as a sleep regulator. The administration of nocturnal GHRH boluses to normal men significantly increased the density of slow-wave sleep, and this has also been shown in other species.212 Furthermore, there is a striking correlation between the age-related declines in slow-wave sleep and daily integrated GH secretion in healthy men.213 These and other data suggest that central GHRH secretion is under circadian entrainment, and nocturnal elevations in GHRH pulse amplitude or frequency directly mediate sleep stage and sleep-induced increases in GH secretion. GHRH has been reported to stimulate food intake in rats and sheep, but the effect is dependent on route of administration, time of administration, and macronutrient composition of the diet.203 The neuropeptide’s physiologic relevance to feeding in humans is unknown, although a study indicated that GHRH stimulated food intake in patients with anorexia nervosa but reduced it in patients with bulimia and in normal female control subjects.214

Growth Hormone–Releasing Peptides In studies of the opioid control of GH secretion, several peptide analogues of met-enkephalin were found to be potent GH secretagogues. These include the GH-releasing peptide GHRP-6 (Fig. 7-21), hexarelin (His-D2MeTrp-AlaTrp-DPhe-Lys-NH2), and other more potent analogues including cyclic peptides and modified pentapeptides.203,215

GHRP– 6: His–DTrp–Ala–Trp–DPhe–Lys–NH2

H H H CH3 CH3 N C C NH2

O O

O

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N H

C

H N O

CH3

CH3 C

C

NH2

O

N CO2CH2CH3

N MK-0677

L-163,540

N CH3SO2

O = C – (CH2)6 – CH3 O Ghrelin: Gly – Ser – Ser – Phe – Leu – Ser – Pro – Glu – His – Gln – Arg – Val – Gln – Gln – Arg – Lys – Glu – Ser – Lys – Lys – Pro – Pro – Ala – Lys – Leu – Gln – Pro – Arg Figure 7-21  Structure of a synthetic peptidyl growth hormone (GH) secretagogue (GH-releasing peptide 6, or GHRP-6) and nonpeptidyl GH secretagogue (MK-0677 and L-163,540) and a natural ligand (ghrelin), all of which bind and activate the growth hormone secretagogue (GHS) receptor. Ghrelin is an acylated 28-amino-acid peptide. The O-n-octanoylation at Ser3 is essential for biologic activity and is a unique post-translational modification among all known proteins. (Adapted from Smith RG, Feighner S, Prendergast K, et al. A new orphan receptor involved in pulsatile growth hormone release. Trends Endocrinol Metab. 1999;10:128-135; Kojima M, Hosoda H, Date Y, et al. Ghrelin is a growth hormone-releasing acylated peptide from stomach. Nature. 1999;402:656-660.)

Neuroendocrinology    137

Subsequently, a series of nonpeptidyl GHRP mimetics were synthesized with greater oral bioavailability, including the spiropiperidine MK-0677 and the shorter-acting benzylpiperidine L-163,540 (see Fig. 7-21). Common to all of these compounds, and the basis of their differentiation from GHRH analogues in pharmacologic activity screens, is their activation of phospholipase C and inositol 1,4,5triphosphate. This property was exploited in a cloning strategy that led to the identification of a G protein– coupled receptor, GHS-R, that is highly selective for the GH secretagogue class of ligands.216 The GHS-R is unrelated to the GHRH receptor and is highly expressed in the anterior pituitary gland and multiple brain areas, including the medial basal hypothalamus, the hippocampus, and the mesencephalic nuclei that are centers of dopamine and serotonin production. Peptidyl and nonpeptidyl GHSs are active when administered by intranasal and oral routes, are more potent on a weight basis than GHRH itself, are more effective in vivo than in vitro, synergize with coadministered GHRH and are almost ineffective in the absence of GHRH, and do not suppress somatostatin secretion.203,208 Prolonged infusions of GHRP amplify pulsatile GH secretion in normal men. GHRP administration, like that of GHRH, facilitates slowwave sleep. Patients with hypothalamic disease leading to GHRH deficiency have low or no response to hexarelin; similarly, pediatric patients with complete absence of the pituitary stalk have no GH secretory response to hexarelin.217 The potent biologic effects of GHRPs and the identification of the GHS-R suggested the existence of a natural ligand for the receptor that is involved in the physiologic regulation of GH secretion. A probable candidate for this ligand is the acylated peptide ghrelin, which is produced and secreted into the circulation from the stomach (Fig. 7-22).13 The effects of ghrelin on GH secretion in humans are identical to or more potent than those of the nonnatural GHRPs (see Fig. 7-20).218 In addition, ghrelin acutely increases circulating PRL, ACTH, cortisol, and aldosterone levels.218 There is debate concerning the extent and localization of ghrelin expression in the brain that must be resolved before the implications of gastric-derived ghrelin in the regulation of pituitary hormone secretion are fully understood. A proposed role for ghrelin in appetite and the regulation of food intake is discussed in Chapter 35.

Clinical Applications GHRH stimulates growth in children with intact pituitaries, but the optimal dosage, route, and frequency of administration, as well as possible usefulness by the nasal route, have not been determined. The availability of recombinant human GH (which requires less frequent injections than GHRH) and the development of the more potent GHSs with improved oral bioavailability have reduced enthusiasm for the clinical use of GHRH or its analogues. GHRH is not useful for the differential diagnosis of hypothalamic and pituitary causes of GH deficiency in children. However, in adults a combined GHRH-GHRP challenge test may be ideal for the diagnosis of GH reserve. GH release in response to the combined secretagogues is not influenced by age, sex, or body mass index, and the test has a wider margin of safety than an insulin tolerance test.219,220 The potential clinical applications of GHSs including MK-0677 are still being explored.203,215 An area of intense interest is the normal decline in GH secretion with age. GH administration in healthy older individuals has been associated with increased lean body mass, increased muscle strength, and decreased fat mass, although there is a high

incidence of adverse side effects. The physiologic GH profile induced by MK-0677 may be better tolerated than GH injections. However, in contrast to treatment with GHRH, chronic administration of GHSs leads to significant desensitization of the GHS-R and attenuation of the GH response. The release of pituitary hormones other than GH may also limit the applicability of GHS therapy. Finally, apart from actions on GH secretion, both GHRH and GHSs are being investigated for the treatment of sleep disorders commonly associated with aging.

Neuroendocrine Regulation of Growth Hormone Secretion GH secretion is regulated by hypothalamic GHRH and somatostatin interacting with circulating hormones and additional modulatory peptides at the level of both the pituitary and the hypothalamus (see Fig. 7-22).203,208,221 Additional background on somatostatin and its functions other than control of GH secretion are presented in a later section (see “Somatostatin”).

Feedback Control Negative feedback control of GH release is mediated by GH itself and by IGF1, which is synthesized in the liver and other tissues under control of GH. Direct GH effects on the hypothalamus are produced by short-loop feedback, whereas those involving IGF1 and other circulating factors influenced by GH, including free fatty acids and glucose, are long-loop systems analogous to the pituitary-thyroid and pituitary-adrenal axes. Control of GH secretion therefore includes two closed-loop systems (GH and IGF1) and one open-loop regulatory system (neural). Although most of the evidence for a direct role of GH in its own negative feedback has been derived from animals, an elegant study in normal men demonstrated that GH pretreatment blocks the subsequent GH secretory response to GHRH by a mechanism that is dependent on somatostatin.222 The mechanism responsible for GH feedback through the hypothalamus has been largely elucidated in rodent models. GH receptors are selectively expressed on somatostatin neurons in the PVH nucleus and on NPY neurons in the arcuate nucleus. Fos gene expression is acutely elevated in both populations of GH receptor–positive neurons by GH administration, indicating an activation of hypothalamic circuitry that includes these neurons. Similarly, GHRH neurons in the arcuate nucleus are acutely activated by MK-0677 because of their selective expression of the GHS-R. Zheng and colleagues223 showed in the latter group of neurons that Fos induction after MK-0677 administration was blocked by pretreatment of mice with GH (Fig. 7-23). The effect must be indirect, because there are no GH receptors on GHRH neurons. However, type 2 somatostatin receptors are expressed on GHRH neurons, and the somatostatin analogue octreotide also significantly blocked Fos activation in the arcuate nucleus by MK-0677. The inhibitory effects of either GH or octreotide pretreatment were abolished in knockout mice lacking the specific somatostatin receptor (see Fig. 7-23). Together with data from many other experiments, these results strongly support a model of GH-negative feedback regulation that involves the primary activation of periventricular somatostatin neurons by GH. These tuberoinfundibular neurons then inhibit GH secretion directly by release of somatostatin in the median eminence, but they also indirectly inhibit GH secretion by way of collateral axonal projections to the arcuate nucleus that synapse on and inhibit GHRH neurons (see Fig. 7-22). It is probable from evidence in rodents that NPY and

138    Neuroendocrinology

GH receptors GHS receptors GHRH receptors SRIF receptors

Raphe nucleus

Periventricular nucleus

SRIF neuron

5-HT

CRH neuron

Brain stem catecholaminergic inputs

Basal forebrain ACh

GHRH neuron NPY neuron SRIF neuron

Arcuate nucleus

DA Galanin

Hypothalamus to CNS

GHRH

Leptin

Fat

SRIF

Ghrelin

Pituitary

FFA

GH

IGF-1 Stomach

Liver Ghrelin

Figure 7-22 Regulation of the hypothalamic-pituitary-growth hormone axis. Growth hormone (GH) secretion by the pituitary is stimulated by growth hormone–releasing hormone (GHRH) and inhibited by somatostatin (SRIF). Negative feedback control of GH secretion is exerted at the pituitary level by insulin-like growth factor type 1 (IGF-1) and by free fatty acids (FFA). GH itself exerts a short-loop negative feedback through activation of SRIF neurons in the hypothalamic periventricular nucleus.These SRIF neurons directly synapse on arcuate GHRH neurons and project axon collaterals to the median eminence. Neuropeptide Y (NPY) neurons in the arcuate nucleus also indirectly modulate GH secretion by integrating peripheral GH, leptin, and ghrelin signals and projecting to periventricular SRIF neurons. Ghrelin is secreted from the stomach and is a putative natural ligand for the growth hormone secretagogue (GHS) receptor that stimulates GH secretion at both the hypothalamic and pituitary levels. On the basis of indirect pharmacologic data, release of GHRH is stimulated by galanin, γ-aminobutyric acid (GABA), α2-adrenergic, and dopaminergic inputs and inhibited by somatostatin. Secretion of somatostatin is inhibited by muscarinic acetylcholine (ACh) and 5-HT-1D receptor ligands and increased by β2-adrenergic stimuli and corticotropin-releasing hormone (CRH). CNS, central nervous system; DA, dopamine; 5-HT, serotonin.

galanin also play a part in the short-loop feedback of GH secretion, but a definitive mechanism in humans is not yet established. IGF1 has a major inhibitory action on GH secretion at the level of the pituitary gland.203 IGF1 receptors are expressed on human somatotroph adenoma cells and inhibit both spontaneous and GHRH-stimulated GH

release. In addition, gene expression of both GH and the pituitary-specific transcription factor PIT1 is inhibited by IGF1. Conflicting data among species suggest that circulating IGF1 may also regulate GH secretion by actions within the brain. The feedback effects of IGF1 account for the fact that serum GH levels are elevated in conditions in which circulating levels of IGF1 are low, such as anorexia nervosa,

Neuroendocrinology    139 50 45

Fos-positive cells/section

40 35 30 25 20 15 10 5 0 Saline/ saline

Saline/ MK

GH/ MK

Octreo/ MK

Sstr2+/+

Saline/ saline

Saline/ MK

GH/ MK

Octreo/ MK

Sstr2–/–

Figure 7-23  Somatostatin and the somatostatin receptor 2 subtype are involved in the short-loop inhibitory feedback of growth hormone (GH) on arcuate neurons. Activation of neurons in the arcuate nucleus was determined by the quantification of immunoreactive Fos-positive cells after administration of the growth hormone secretagogue MK-0677 (MK). Preliminary treatment of wild-type mice (SstR2+/+) with either GH or the somatostatin analogue octreotide (Octreo) significantly attenuated the neuronal activation by MK-0677. In contrast, GH and octreotide had no effect on MK-0677 neuronal activation in somatostatin receptor 2-deficient mice (SstR2−/−). (Adapted from Zheng H, Bailey A, Jian M-H, et al. Somatostatin receptor subtype 2 knockout mice are refractory to growth hormone-negative feedback on arcuate neurons. Mol Endocrinol. 1997;11:1709-1717.)

protein-calorie starvation,224 and Laron dwarfism (the result of a defect in the GH receptor).

Neural Control The predominant hypothalamic influence on GH release is stimulatory, and section of the pituitary stalk or lesions of the basal hypothalamus cause reduction of basal and induced GH release.203 When the somatostatinergic component is inactivated (e.g., by anti-somatostatin antibody injection in rats), basal GH levels and GH responses to the usual provocative stimuli are enhanced. GHRH-containing nerve fibers that terminate adjacent to portal vessels in the external zone of the median eminence arise principally from within, above, and lateral to the infundibular nucleus in human hypothalamus, corresponding to rodent arcuate and ventromedial nuclei.225 Perikarya of the tuberoinfundibular somatostatin neurons are located almost completely in the medial periventricular nucleus and the parvicellular component of the anterior PVH. Neuroanatomic and functional evidence suggests a bidirectional synaptic interaction between the two peptidergic systems.203 Multiple extrahypothalamic brain regions provide efferent connections to the hypothalamus and regulate GHRH and somatostatin neuronal activity (Fig. 7-24; see Fig. 7-22). Somatosensory and affective information is integrated and filtered through the amygdaloid complex. The basolateral amygdala provides an excitatory input to the hypothalamus, and the central extended amygdala, which includes the central and medial nuclei of the amygdala together with the bed nucleus of the stria terminalis, provides a GABAergic inhibitory input. Many intrinsic neurons

of the hypothalamus also release GABA, often with a peptide cotransmitter. Excitatory cholinergic fibers arise to a small extent from forebrain projection nuclei but mostly from hypothalamic cholinergic interneurons, which densely innervate the external zone of the median eminence. Similarly, the origin of dopaminergic and histaminergic neurons is local, with their cell bodies located in the hypothalamic arcuate and tuberomammillary bodies, respectively. Two important ascending pathways to the medial basal hypothalamus regulate GH secretion and originate from serotoninergic neurons in the raphe nuclei and adrenergic neurons in the nucleus of the tractus solitarius and ventral lateral nucleus of the medulla. Both GHRH and somatostatin neurons express presynaptic and postsynaptic receptors for multiple neurotransmitters and peptides (Table 7-5). The α2-adrenoreceptor agonist clonidine reliably stimulates GH release, and for this reason a clonidine test was a standard diagnostic tool in pediatric endocrinology. The stimulatory effect is blocked by the specific α2-antagonist yohimbine and appears to involve a dual mechanism of action—inhibition of somatostatin neurons and activation of GHRH neurons. In addition, partial attenuation of the effects of clonidine by mixed serotonin 5-HT1 and 5-HT2 antagonists suggests that some of the relevant α2-receptors are located presynaptically on serotoninergic nerve terminals and increase serotonin release. Both norepinephrine and epinephrine play physiologic roles in the adrenergic stimulation of GH secretion. The α1-agonists have no effect on GH secretion in humans, but β2-agonists such as the bronchodilator salbutamol inhibit GH secretion by stimulating the release of somatostatin from nerve terminals in the median

140    Neuroendocrinology

Psychological stress

Limbic BNST/ Amyg SCN

Sleep stage

NTS

SRIF GHRH

Hormonal signals Cytokines Metabolic signals

HYP

Raphe

VLM

GH

Figure 7-24  Neural pathways involved in growth hormone (GH) regulation. This diagram illustrates the varied pathways by which impulses from the limbic system and brain stem ultimately impinge on the hypothalamic periventricular and arcuate nuclei to stimulate GH release through the mediation of somatostatin (SRIF) and growth hormone–releasing hormone (GHRH). Psychological stress modulates hypothalamic function indirectly through the bed nucleus of the stria terminalis (BNST) and the amygdalar complex (Amyg). Circadian rhythms are entrained in part by projections from the suprachiasmatic nucleus (SCN). Complex reciprocal interactions between sleep stage and GHRH release involve cortex and subcortical nuclei, but the detailed mechanisms are not known. Dopaminergic and histaminergic input are from neurons located in the arcuate and mammillary nuclei, respectively, of the hypothalamus (HYP). Ascending catecholaminergic projections arise in both the nucleus of the tractus solitarius (NTS) and the ventral lateral medulla (VLM). Serotoninergic (5-HT) afferents are from the raphe nuclei. In addition to these neural pathways, a variety of peripheral hormonal and metabolic signals and cytokines influence GH secretion by actions within the medial basal hypothalamus and pituitary gland.

eminence. These effects are blocked by propranolol, a nonspecific β-antagonist. Dopamine generally has a net effect of stimulation of GH secretion, but the mechanism is not clear because of multiple dopamine receptor subtypes and the apparent activation of both GHRH and somatostatin neurons. Serotonin’s effect on GH release in humans was difficult to decipher because of the large number of receptor subtypes. However, clinical studies with the receptor-selective agonist sumatriptan clearly implicated the 5-HT1D receptor subtype in the stimulation of basal GH levels.226 The drug also potentiates the effect of a maximal dose of GHRH, suggesting the recurring theme of GH disinhibition by inhibition of hypothalamic somatostatin neurons in its mechanism of action. Histaminergic pathways acting through H1 receptors play only a minor, conditional stimulatory role in GH secretion in humans. Acetylcholine appears to be an important physiologic regulator of GH secretion.227 Blockade of muscarinic acetylcholine receptors reduces or abolishes GH secretory responses to GHRH, glucagon and arginine, morphine, and exercise. In contrast, drugs that potentiate cholinergic transmission increase basal GH levels and enhance the GH response to GHRH in normal individuals and in subjects with obesity or Cushing’s disease. In vitro acetylcholine inhibits somatostatin release from hypothalamic fragments, and acetylcholine can act directly on the pituitary to inhibit GH release. There may even be a paracrine cholinergic control system within the pituitary. However, the sum of evidence suggests that the primary mechanism of action of muscarinic M1 agonists is inhibition of somatostatin neuronal activity or release of peptide from somatostatinergic terminals. Short-term cholinergic blockade with the M1 muscarinic receptor antagonist pirenzepine reduced the GH excess in patients with poorly controlled diabetes mellitus.228 However, in the long term, cholinergic blockade did not prevent complications associated with the hypersomatotropic state. Many neuropeptides in addition to GHRH and somatostatin are involved in the modulation of GH secretion in

humans (see Table 7-5).203,208 Among these, the evidence is most compelling for a stimulatory role of galanin acting in the human hypothalamus by a GHRH-dependent mechanism.229 Many GHRH neurons are immunopositive for galanin as well as neurotensin and tyrosine hydroxylase. Galanin’s actions may be explained, in part, by presynaptic facilitation of catecholamine release from nerve terminals and subsequent direct adrenergic stimulation of GHRH release.230 Opioid peptides also stimulate GH release, probably by disinhibition of GHRH neurons, but under normal circumstances, endogenous opioid tone in the hypothalamus is presumed to be low because opioid antagonists have little acute effect on GH secretion. A larger number of neuropeptides are known or suspected to inhibit GH secretion in humans, at least under certain circumstances.208 The list includes NPY, CRH, calcitonin, oxytocin, neurotensin, VIP, and TRH. Inhibitory actions of NPY are well established in the rat. The effect on GH secretion is secondary to stimulation of somatostatin neurons and is of particular interest because of the presumed role in GH autofeedback (discussed earlier) and the integration of GH secretion with regulation of energy intake and expenditure (see “External and Metabolic Signals”). Finally, TRH has the well-established paradoxical effect of increasing GH secretion in patients with acromegaly, type 1 diabetes mellitus, hypothyroidism, or hepatic or renal failure.

Other Factors Influencing Secretion of Growth Hormone Human Growth Hormone Rhythms.  The deciphering of rhythmic GH secretion has relied on a combination of technical innovations in sampling and GH assay and sophisticated mathematical modeling, including deconvolution analysis and the calculation of approximate entropy as a measure of orderliness or regularity in minute-tominute secretory patterns.208 At least three distinct categories of GH rhythms, which differ markedly in their time scales, can be considered here.

Neuroendocrinology    141 TABLE 7-5 

Factors That Change Growth Hormone (GH) Secretion in Humans Physiologic

Hormones and Neurotransmitters

Pathologic

Stimulatory Factors Episodic, spontaneous release Exercise Stress   Physical   Psychological Slow-wave sleep Postprandial glucose decline Fasting

Insulin hypoglycemia 2-Deoxyglucose Amino acid infusions   Arginine, lysine Neuropeptides   GHRH   Ghrelin   Galanin   Opioids (µ-receptors)   Melatonin Classic neurotransmitters   α2-Adrenergic agonists   β-Adrenergic antagonists   M1 cholinergic agonists   5-HT1D-serotonin agonists   H1-histamine agonists   GABA (basal levels)   Dopamine (? D2 receptor)   Estrogen   Testosterone   Glucocorticoids (acute)

Acromegaly   TRH   GnRH   Glucose   Arginine Interleukins 1, 2, 6 Protein depletion Starvation Anorexia nervosa Renal failure Liver cirrhosis Type 1 diabetes mellitus

Glucose infusion Neuropeptides   Somatostatin   Calcitonin   Neuropeptide Y (NPY†)   CRH† Classic neurotransmitters   α1/2-Adrenergic antagonists   β2-Adrenergic agonists   H1 histamine antagonists   Serotonin antagonist   Nicotinic cholinergic agonists Glucocorticoids (chronic)

Acromegaly L-Dopa D2R DA agonists Phentolamine Galanin Obesity Hypothyroidism Hyperthyroidism

Inhibitory Factors* Postprandial hyperglycemia Elevated free fatty acids Elevated GH levels Elevated IGF1 (pituitary) REM sleep Senescence, aging

*In many instances, the inhibition can be demonstrated only as a suppression of GH release induced by a pharmacologic stimulus. † The inhibitory actions of NPY and CRH on GH secretion are firmly established in the rodent and are secondary to increased somatostatin tone. Contradictory evidence exists in the human for both peptides, and further studies are required. CRH, Corticotropin-releasing hormone; DA, dopamine; GHRH, growth hormone–releasing hormone; GnRH, gonadotropin-releasing hormone; IGF1, insulin-like growth factor type 1; REM, rapid eye movement;TRH, thyrotropinreleasing hormone.

The daily GH secretion rate varies over 2 orders of magnitude, from a maximum of almost 2.0 mg/day in late puberty to a minimum of 20 µg/day in older or obese adults. The neonatal period is characterized by markedly amplified GH secretory bursts followed by a prepubertal decade of stable, moderate GH secretion of 200 to 600 µg/ day. There is a marked increase in daily GH secretion during puberty that is accompanied by a commensurate

rise in plasma IGF1 to levels that constitute a state of physiologic hypersomatotropism. This pubertal increase in GH secretion is the result of increased GH mass per secretory burst rather than increased pulse frequency. Although the changes are clearly related to the increases in gonadal steroid hormones and can be mimicked by administration of estrogen or testosterone to hypogonadal children, the underlying neuroendocrine mechanisms are not fully understood. One hypothesis is that decreased sensitivity of the hypothalamic-pituitary axis to negative feedback from GH and IGF1 leads to increased GHRH release and action. Young adults have a return of daily GH secretion to prepubertal levels despite continued gonadal steroid elevation. The so-called somatopause is defined by an exponential decline in GH secretory rate with a half-life of 7 years starting in the third decade of life. GH secretion in young adults exhibits a true circadian rhythm over a 24-hour period, characterized by a greater nocturnal secretory mass that is independent of sleep onset.231 However, as discussed earlier, GH release is further facilitated when slow-wave sleep coincides with the normal circadian peak. Under basal conditions, GH levels are low most of the time, with an ultradian rhythm of about 10 secretory pulses per 24 hours in men (20 in women), as calculated by deconvolution analysis.232 Both sexes have an increased pulse frequency during the nighttime hours, but the fraction of total daily GH secretion associated with nocturnal pulses is much greater in men. Overall, women have more continuous GH secretion and more frequent GH pulses that are of more uniform size than in men.232 A complementary study using approximate entropy analysis concluded that the nonpulsatile regularity of GH secretion is also significantly different in men and women.233 These sexually dimorphic patterns in the human are actually quite similar to those in the rat, although the sex differences are not as extreme in humans.208,233 The neuroendocrine basis for sex differences in the ultradian rhythm of GH secretion is not fully understood. Gonadal sex steroids play both an organizational role during development of the hypothalamus and an activational role in the adult, regulating expression of the genes for many of the peptides and receptors central to GH regulation.203,208 In the human, unlike the rat, the hypothalamic actions of testosterone appear to result predominantly from its aromatization to 17β-estradiol and interaction with estrogen receptors. Hypothalamic somatostatin ap­ pears to play a more prominent role in men than in women in the regulation of pulsatile GH secretion, and this difference is postulated to be a key factor in producing the sexual dimorphism.232,234,235 External and Metabolic Signals.  The various peripheral signals that modulate GH secretion in humans are summarized in Table 7-5 (also see Figs. 7-22 and 7-24). Of particular importance are factors related to energy intake and metabolism, because they provide a common signal between the peripheral tissues and hypothalamic centers regulating nonendocrine homeostatic pathways in addition to the classic hypophyseotropic neurons. It is also in this complex arena that species-specific regulatory responses are particularly prominent, making extrapolations between rodent experimental models and human GH regulation less reliable.203,208 Important triggers of GH release include the normal decrease in blood glucose concentration after intake of a carbohydrate-rich meal, absolute hypoglycemia, exercise, physical and emotional stress, and high intake of protein

142    Neuroendocrinology (mediated by amino acids). Some of the pathologic causes of elevated GH represent extremes of these physiologic signals and include protein-calorie starvation, anorexia nervosa, liver failure, and type 1 diabetes mellitus. A critical concept is that many of these GH triggers work through the same final common mechanism of somatostatin withdrawal and consequent disinhibition of GH secretion. In contrast, postprandial hyperglycemia, glucose infusion, elevated plasma free fatty acids, type 2 diabetes mellitus (with obesity and insulin resistance), and obesity are all associated with inhibition of GH secretion. The role of leptin in mediating increases or decreases in GH release is complicated by its multiple sites of action and coexistent secretory environment. Similarly, other members of the cytokine family including IL-1, IL-2, IL-6, and endotoxin have been inconsistently shown to stimulate GH in humans. The actions of steroid hormones on GH secretion are complex because of their multiple loci of action within the proximal hypothalamic-pituitary components in addition to secondary effects on other neural and endocrine systems. Glucocorticoids in particular produce opposite responses that are dependent on the chronicity of administration. Moreover, glucocorticoid effects follow an inverted U-shaped dose-response curve. Both low and high glucocorticoid levels reduce GH secretion, the former because of decreased GH gene expression and somatotroph responsiveness to GHRH and the latter because of increased hypothalamic somatostatin tone and decreased GHRH. Similarly, physiologic levels of thyroid hormones are necessary to maintain GH secretion and promote GH gene expression. Excessive thyroid hormone is also inhibitory to the GH axis, and the mechanism is speculated to be a combination of increased hypothalamic somatostatin tone, GHRH deficiency, and suppressed pituitary GH production.

Somatostatin Chemistry and Evolution A factor that potently inhibits GH release from pituitary in vitro was unexpectedly identified during early efforts to isolate GHRH from hypothalamic extracts.236 Somatostatin, the peptide responsible for this inhibition of GH secretion and the inhibition of insulin secretion by a pancreatic islet extract, was eventually isolated from hypothalamus and sequenced by Brazeau and colleagues in 1973.237 The term somatostatin was originally applied to a cyclic peptide containing 14 amino acids, called somatostatin-14 (SST-14) (Fig. 7-25). Subsequently, a second form, known as Nterminal extended somatostatin-28 (SST-28), was identified as a secretory product. Both forms of somatostatin are derived through independent cleavage of a common prohormone by prohormone convertases.238 In addition, the isolation of SST-28(1-12) in some tissues suggests that SST-14 can be secondarily processed from SST-28. SST-14 is the predominant form in the brain (including the hypothalamus), whereas SST-28 is the major form in the gastrointestinal tract, especially the duodenum and jejunum. The name somatostatin is descriptively inadequate because the molecule also inhibits TSH secretion from the pituitary and has nonpituitary roles including activity as a neurotransmitter or neuromodulator in the central and peripheral nervous systems and as a regulatory peptide in gut and pancreas. As a pituitary regulator, somatostatin is

a true neurohormone—that is, a neuronal secretory product that enters the blood (hypophyseal-portal circulation) to affect cell function at remote sites. In the gut, somatostatin is present in the myenteric plexus, where it acts as a neurotransmitter, and in epithelial cells, where it influences the function of adjacent cells as a paracrine secretion. Somatostatin can influence its own secretion from delta cells (an autocrine function) in addition to acting as a paracrine factor in pancreatic islets. Gut exocrine secretion can be modulated by intraluminal action, so it is also a lumone. Because of its wide distribution, broad spectrum of regulatory effects, and evolutionary history, this peptide can be regarded as an archetypical pansystem modulator. The genes that encode somatostatin in humans239 (see Fig. 7-25) and a number of other species exhibit striking sequence homology, even in primitive fish such as the anglerfish. Furthermore, the amino acid sequence of SST-14 is identical in all vertebrates. Formerly, it was accepted that all tetrapods have a single gene encoding both SST-14 and SST-28 and that teleost fish have two nonallelic preprosomatostatin genes (PPSI and PPSII), each of which encodes only one form of the mature somatostatin peptides. This situation implied that a common ancestral gene underwent a duplication event after the split of teleosts from the descendants of tetrapods. However, both lampreys and amphibians, which predate and postdate the teleost evolutionary divergence, respectively, have now been shown to have at least two PPS genes.240 A more distantly related gene has been identified in mammals that encodes cortistatin, a somatostatin-like peptide that is highly expressed in cortex and hippocampus.241,242 Cortistatin-14 differs from SST-14 by three amino acid residues but has high affinity for all known subtypes of somatostatin receptors (see later discussion). The human gene sequence predicts a tripeptide-extended cortistatin-17 and a further N-terminal extended cortistatin-29.243 A revised evolutionary concept of the somatostatin gene family is that a primordial gene underwent duplication at or before the advent of chordates, and the two resulting genes underwent mutation at different rates to produce the distinct pre-prosomatostatin and pre-procortistatin genes in mammals.240 A second gene duplication probably occurred in teleosts to generate PPSI and PPSII from the ancestral somatostatin gene. Apart from its expression in neurons of the PVH and the arcuate hypothalamic nucleus and its involvement in GH secretion (discussed earlier), somatostatin is highly expressed in the cortex, lateral septum, extended amygdala, reticular nucleus of the thalamus, hippocampus, and many brain stem nuclei. Cortistatin is present in the brain, at a small fraction of the level of somatostatin and in a more limited distribution, primarily confined to cortex and hippocampus. The molecular mechanisms underlying the developmental and hormonal regulation of somatostatin gene transcription have been most extensively studied in pancreatic islet cells.244-246 Less is known concerning the regulation of somatostatin gene expression in neurons, except that activation is strongly controlled by binding of the phosphorylated transcription factor CRE-binding protein to its cognate CRE contained in the promoter sequence.247,248 Enhancer elements in the somatostatin gene promoter that bind complexes of homeodomaincontaining transcription factors (PAX6, PBX, PREP1) and upregulate gene expression in pancreatic islets may actually represent gene silencer elements in neurons (see Fig. 7-25, promoter elements TSEII and UE-A).246 Conversely,

UE-A

CRE TATA

TSE I

TSE II

50 bp

Intron

Exon 1

Exon 2 SST-28

5'UTR Signal Peptide

+5 0

-3 00

Neuroendocrinology    143

3'UTR

200 bp Poly A

SST-14

100 bp

PC1/PC2 CPE SST-28

Ser – Ala – Asn – Ser – Asn – Pro – Ala – Met – Ala – Pro – Arg – Glu – Arg – Lys – Ala – Gly – Cys – Lys – Asn – Phe – Phe – Trp – Lys – Thr – Phe – Thr – Ser – Cys

SST-28(1-12) SST-14

Ser – Ala – Asn – Ser – Asn – Pro – Ala – Met – Ala – Pro – Arg – Glu Ala – Gly – Cys – Lys – Asn – Phe – Phe S

Trp

S

Lys

Cys – Ser – Thr – Phe – Thr Octreotide

DPhe

– Cys – Phe DTrp

S S OL – Thr – Cys –

Lys Thr

Figure 7-25 Diagram illustrating the genomic organization, messenger RNA structure, and post-translational processing of the human somatostatin prohormone. Transcriptional regulation of the somatostatin gene has been studied extensively in pancreatic islet cell lines. Binding sites for specific factors, including tissue-specific elements (TSE), upstream elements (UE), and the cyclic adenosine monophosphate (cAMP) response element (CRE), have been identified. It is not known whether all or some of these factors are also involved in the neural-specific expression of somatostatin. SST-28 and SST-14 are cyclic peptides that contain a single covalent disulfide bond between a pair of Cys residues. A β-turn containing the tetrapeptide Phe-Trp-Lys-Thr is stabilized by hydrogen bonds to produce the core receptor binding epitope. This minimal structure has been the model for conformationally restrained analogues of somatostatin including octreotide. CPE, carboxypeptidase E; PC1/PC2, prohormone convertases 1 and 2; SST, somatostatin; TATA, Goldstein-Hogness box involved in binding RNA polymerase; UTR, untranslated region. (Compiled from data by Shen LP, Rutter WJ. Sequence of the human somatostatin 1 gene. Science. 1984;224:168-171; Goudet G, Delhalle S, Biemar F, et al. Functional and cooperative interactions between the homeodomain PDX1, Pbx, and Prep1 factors on the somatostatin promoter. J Biol Chem. 1999;274:4067-4073; and Milner-White EJ. Predicting the biologically active conformations of short polypeptides. Trends Pharmacol Sci. 1989;10:70-74.)

another related cis element in the somatostatin gene (see Fig. 7-25, promoter element TSEI) apparently binds a homeodomain transcription factor PDX1 (also called STF1, IDX1, or IPF1) that is common to developing brain, pancreas, and foregut and regulates gene expression in both the CNS and the gut.249 The function of somatostatin in GH and TSH regulation was considered earlier in this chapter. Its actions in the extrahypothalamic brain and diagnostic and therapeutic roles are considered in the remainder of this section and in Chapter 8. An additional function of somatostatin in pancreatic islet cell regulation is described in Chapter 34, and the manifestations of somatostatin excess (as in somatostatinoma) are described in Chapter 39.

Somatostatin Receptors Five somatostatin receptor subtypes (SSTR1 to SSTR5) have been identified by gene cloning techniques, and one of these (SSTR2) is expressed in two alternatively spliced forms.250 These subtypes are encoded by separate genes located on different chromosomes; they are expressed in unique or partially overlapping distributions in multiple target organs; and they differ in their coupling to secondmessenger signaling molecules and therefore in their range and mechanism of intracellular actions.250,251 The subtypes also differ in their binding affinity to specific somatostatin analogues. Certain of these differences have important implications for the use of somatostatin analogues in therapy and in diagnostic imaging.

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Biologic Actions of Somatostatin Outside the Central Nervous System Hormone Secretion Inhibited (by Gland) Pituitary gland   GH, thyrotropin, ACTH, prolactin Gastrointestinal tract   Gastrin   Secretin   Gastrointestinal polypeptide   Motilin   Glicentin (enteroglucagon)   Vasoactive intestinal peptide Pancreas   Insulin   Glucagon   Somatostatin Genitourinary tract   Renin

Other Gastrointestinal and Extragastrointestinal Actions Inhibited Gastric acid secretion Gastric and jejunal fluid secretion Gastric emptying Pancreatic bicarbonate secretion Pancreatic enzyme secretion Stimulates intestinal absorption of water and electrolytes Gastrointestinal blood flow AVP-stimulated water transport Bile flow Extragastrointestinal Actions Inhibits the function of activated immune cells Inhibition of tumor growth

ACTH, adrenocorticotropic hormone; AVP, arginine vasopressin; GH, growth hormone.

hormone and calcitonin. Somatostatin blocks hormone release in many endocrine-secreting tumors, including insulinomas, glucagonomas, VIPomas, carcinoid tumors, and some gastrinomas. The physiologic actions of somatostatin in extrahypothalamic brain remain the subject of investigation.253 In the striatum, somatostatin increases the release of dopamine from nerve terminals by a glutamate-dependent mechanism. It is widely expressed in GABAergic interneurons of limbic cortex and hippocampus, where it modulates the excitability of pyramidal neurons. Temporal lobe epilepsy is associated with a marked reduction in somatostatin-expressing neurons in the hippocampus, consistent with a putative inhibitory action on seizures.254 A wealth of correlative data has linked reduced forebrain and CSF concentrations of somatostatin with Alzheimer’s disease, major depression, and other neuropsychiatric disorders, raising speculation about the role of somatostatin in modulating neural circuits underlying cognitive and affective behaviors. A study using both genetic and pharmacologic methods to induce somatostatin deficiency in mice bolstered the hypothesis that the neuropeptide plays a physiologic role in the acquisition of contextual fear memory, possibly by altering long-term potentiation in hippocampal circuits.255

Clinical Applications of Somatostatin Analogues All SSTR subtypes are coupled to pertussis toxin–sensitive G proteins and bind SST-14 and SST-28 with high affinity in the low nanomolar range, although SST-28 has a uniquely high affinity for SSTR5. SSTR1 and SSTR2 are the two most abundant subtypes in brain; they probably function as presynaptic autoreceptors in the hypothalamus and limbic forebrain, respectively, in addition to their postsynaptic actions. SSTR4 is most prominent in hippocampus. All the subtypes are expressed in pituitary, but SSTR2 and SSTR5 are the most abundant subtypes on somatotrophs. They are also the most physiologically important subtypes in pancreatic islets, with SSTR5 responsible for inhibition of insulin secretion from beta cells and SSTR2 responsible for inhibition of glucagon from alpha cells.252 Binding of somatostatin to its receptor leads to activation of one or more plasma membrane–bound inhibitory G proteins (Gi/o), which in turn inhibit adenylyl cyclase activity and lower intracellular cAMP. Other G protein–mediated actions common to all SSTRs are acti­ vation of a vanadate-sensitive phosphotyrosine phosphatase and modulation of mitogen-activated protein kinase (MAPK). Different subsets of SSTRs are also coupled to inwardly rectifying K+ channels, voltage-dependent Ca2+ channels, an Na+/H+ exchanger, α-amino-3-hydroxy-5methyl-4-isoxazole proprionic acid (AMPA)-kainate glutamate receptors, phospholipase C, and phospholipase A2.250 The lowering of intracellular cAMP and Ca2+ is the most important mechanism for inhibition of hormone secretion, and actions on phosphotyrosine phosphatase and MAPK are postulated to play a role in somatostatin’s antiproliferative effect on tumor cells.

Effects on Target Tissues and Mechanisms of Action In the pituitary, somatostatin inhibits secretion of GH, TSH, and, under certain conditions, PRL and ACTH. It exerts inhibitory effects on virtually all endocrine and exocrine secretions of the pancreas, gut, and gallbladder (Table 7-6). Somatostatin inhibits secretion by the salivary glands and, under some conditions, secretion of parathyroid

An extensive pharmaceutical discovery program has produced somatostatin analogues with receptor subtype selectivity and improved pharmacokinetics and oral bioavailability compared with the native peptide. Initial efforts focused on the rational design of constrained cyclic peptides incorporating D-amino acid residues and including the Trp8-Lys9 dipeptide of somatostatin, which was shown by structure-function studies to be necessary for high-affinity binding to the somatostatin receptor (see Fig. 7-25). Many such analogues have been studied in clinical trials, including octreotide, lanreotide, vapreotide, and the hexapeptide MK-678. These compounds are agonists with similar high-affinity binding to SSTR2 and SSTR5, moderate binding to SSTR3, and no (or low) binding to SSTR1 and SSTR4. A combinatorial chemistry approach has now led to a new generation of nonpeptidyl somatostatin agonists that bind selectively and with subnanomolar affinity to each of the five SSTR subtypes.256,257 In contrast to the marked success in development of potent and selective somatostatin agonists, there is a relative paucity of useful antagonists.250 The actions of octreotide (SMS 201-995 or Sandostatin) illustrate the general therapeutic potential of somatostatin analogues.258,259 Octreotide controls excess secretion of GH in acromegaly in most patients and shrinks tumor size in about one third. It is also indicated for the treatment of recurrent TSH-secreting adenomas after surgery. It is used to treat other functioning metastatic neuroendocrine tumors, including carcinoid, VIPoma, glucagonoma, and insulinoma, but is seldom of use for the treatment of gastrinoma. Octreotide is also useful in the management of many forms of diarrhea (acting on salt and water excretion mechanisms in the gut) and in reducing external secretions in pancreatic fistulas (thus permitting healing). A decrease in blood flow to the gastrointestinal tract is the basis for its use in bleeding esophageal varices, but it is not effective in the treatment of bleeding from a peptic ulcer. The only major undesirable side effect of octreotide is reduction of bile production and of gallbladder contractility, which leads to “sludging” of bile and an increased incidence of gallstones. Other common adverse effects,

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Figure 7-26 The use of 111In-labeled diethylenetriaminepentaacetic acid (DTPA)-octreotide (radioactive somatostatin analogue) and external imaging techniques to localize a carcinoid tumor expressing somatostatin receptors. Scans were obtained 24 hours after administration of labeled tracer. A, Anterior view of the abdomen showing nodular metastases in an enlarged liver and the primary carcinoid tumor (arrow) in the wall of the jejunum of a patient with severe flushing and diarrhea. B, Posterior view of the chest and neck showing a metastasis in a lymph node on the left side of the neck (arrow) and multiple metastases in the ribs and pleura. (Reprinted with modifications from Lamberts SWJ, Krenning EP, Reubi J-C. The role of somatostatin and its analogs in the diagnosis and treatment of tumors. Endocrine Rev. 1991;12:450-482. Copyright 1991, The Endocrine Society.)

A

including nausea, abdominal cramps, diarrhea secondary to malabsorption of fat, and flatulence, usually subside spontaneously within 2 weeks of continued treatment. Impaired glucose tolerance is not associated with long-term octreotide therapy, despite an inhibitory effect on insulin secretion, because of compensating reductions in carbohydrate absorption and GH and glucagon secretion that are caused by the drug. Somatostatin analogues labeled with a radioactive tracer have been used as external imaging agents for a wide range of disorders.258,259 An indium 111 (111In)-labeled analogue of octreotide (OctreoScan) has been approved for clinical use in the United States and several other countries (Fig. 7-26). The majority of neuroendocrine tumors and many pituitary tumors that express somatostatin receptors are visualized by external imaging techniques after administration of this agent; a variety of nonendocrine tumors and inflammatory lesions are also visualized, all of which have in common the expression of somatostatin receptors. Such tumors include non–small cell cancer of the lung (100%), meningioma (100%), breast cancer (74%), and astrocytomas (67%). Because activated T cells of the immune system display somatostatin receptors, inflammatory lesions that take up the tracer include sarcoidosis, Wegener’s granulomatosis, tuberculosis, and many cases of Hodgkin’s disease and non-Hodgkin’s lymphoma. Although the tracer lacks specificity in differential diagnosis, its ability to identify the presence of abnormality and the extent of the lesion provides important information for management, including tumor staging. The use of a small, hand-held radiation detector in the operating room makes it possible to ensure the completeness of removal of medullary thyroid carcinoma metastases.260 New developments in the synthesis of tracers chelated to octreotide for positron emission tomography have allowed the sensitive detection of meningiomas only 7 mm in diameter and located beneath osseous structures at the base of the skull.261 The ability of somatostatin to inhibit the growth of normal and some neoplastic cell lines and to reduce the growth of experimentally induced tumors in animal models has stimulated interest in somatostatin analogues for the treatment of cancer. Somatostatin’s tumoristatic effects may be a combination of direct actions on tumor cells related to inhibition of growth factor receptor expression, inhibition of MAPK, and stimulation of phosphotyrosine phosphatase. SSTR1, SSTR2, SSTR4, and SSTR5 can all promote cell cycle arrest associated with induction of the tumor suppressor retinoblastoma (Rb) and p21 (CDKN1A),

B

and SSTR3 can trigger apoptosis accompanied by induction of the tumor suppressor TP53 and the proapoptotic protein Bax.250 In addition, somatostatin has indirect effects on tumor growth through inhibition of circulating, paracrine, and autocrine tumor growth–promoting factors, and it can modulate the activity of immune cells and influence tumor blood supply. Despite this promise, the therapeutic util­ ity of octreotide as an antineoplastic agent remains controversial. Two new treatment approaches in preclinical trials may yet effectively utilize somatostatin receptors in the arrest of cancer cells.258 The first is receptor-targeted radionuclide therapy using octreotide chelated to a variety of β- or γ-emitting radioisotopes. Theoretical calculations and empiric data suggest that radiolabeled somatostatin analogues can deliver a tumoricidal radiotherapeutic dose to some tumors after receptor-mediated endocytosis. A variation on this theme is the chelation of a cytotoxic chemotherapeutic agent, such as doxorubicin, to a somatostatin analogue. A second approach involves somatic cell gene therapy to transfect SSTR-negative pancreatic cancer cells with an SSTR gene.262 Therapeutic results can be obtained with the creation of autocrine or paracrine inhibitory growth effects or the addition of targeted radionuclide treatments.

Prolactin-Regulating Factors Dopamine It is well known that PRL secretion, unlike the secretion of other pituitary hormones, is primarily under tonic inhibitory control by the hypothalamus (Fig. 7-27).263 Destruction of the stalk median eminence or transplantation of the pituitary gland to ectopic sites causes a marked constitutive increase in PRL secretion, in contrast to a decrease in the release of GH, TSH, ACTH, and the gonadotropins. Many lines of evidence indicate that dopamine is the principal physiologic PIF released from the hypothalamus.264 Dopamine is present in hypophyseal–portal vessel blood in sufficient concentration to inhibit PRL release265; dopamine inhibits PRL secretion from lactotrophs both in vivo and in vitro266; and dopamine D2 receptors are expressed on the plasma membrane of lactotrophs.267,268 Mutant mice with a targeted disruption of the D2 receptor gene (Drd2) uniformly developed lactotroph hyperplasia, hyper­prolactinemia, and eventually lactotroph adenomas, further emphasizing the importance of dopamine in the

146    Neuroendocrinology

Paraventricular nucleus

Hypothalamic glutamatergic inputs

Basal forebrain

5-HT

Raphe nucleus

PRF neurons

Hypothalamic opioid neurons Mammillary nuclei Histamine

Dopamine neuron

ACh Arcuate nucleus GABA

Hypothalamus

Estrogen

Dopamine GABA

TRH Oxytocin VIP

to CNS Pituitary

PRL

Spinal afferent

Breast PRL receptors Estrogen receptors

Suckling stimulus Multiple target organs

Figure 7-27 Regulation of the hypothalamic-pituitary-prolactin (PRL) axis. The predominant effect of the hypothalamus is inhibitory, an effect mediated principally by the tuberohypophyseal dopaminergic neuron system and dopamine D2 receptors on lactotrophs. The dopamine neurons are stimulated by acetylcholine (ACh) and glutamate and inhibited by histamine and opioid peptides. One or more prolactin-releasing factors (PRFs) probably mediate acute release of PRL (e.g., in suckling, during stress). There are several candidate PRFs, including thyrotropin-releasing hormone (TRH), vasoactive intestinal polypeptide (VIP), and oxytocin. PRF neurons are activated by serotonin (5-HT). Estrogen sensitizes the pituitary to release PRL, which feeds back on the pituitary to regulate its own secretion (ultrashort-loop feedback) and also influences gonadotropin secretion by suppressing the release of luteinizing hormone–releasing hormone (LHRH). Short-loop feedback is also mediated indirectly by PRL receptor regulation of hypothalamic dopamine synthesis, secretion, and turnover. CNS, central nervous system; GABA, γ-aminobutyric acid.

physiologic regulation of lactotroph proliferation in addition to hormone secretion.269 The intrinsic dopamine neurons of the medial-basal hypothalamus constitute a dopaminergic population with regulatory properties that are distinct from those in other areas of the brain. Notably, they lack D2 autoreceptors but express PRL receptors, which are essential for positive feedback control (discussed in detail later). In the rat, these

neurons are subdivided by location into the A12 group within the arcuate nucleus and the A14 group in the anterior PVH. The caudal A12 dopamine neurons are further described as tuberoinfundibular (TIDA) because of their axonal projections to the external zone of the median eminence. Tuberohypophyseal (THDA) neuronal soma are located more rostrally in the arcuate nucleus and project to both the neural lobe and the intermediate lobe through

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axon collaterals that are found in the internal zone of the median eminence. Finally, the A14 periventricular hypo­ physeal (PHDA) neurons send their axons only to the intermediate lobe of the pituitary gland. Although the TIDA neurons are considered to be the major source of dopamine to the anterior lobe through the long portal vessels originating in the median eminence, dopamine can also reach the anterior lobe from the neural and intermediate lobes by the interconnecting short portal veins.270 Consistent with this pathway for dopamine access to the anterior lobe, surgical removal of the neurointermediate lobe in rats caused a significant increase in basal PRL levels.271 In addition to direct actions of dopamine on lactotrophs, central dopamine can indirectly affect PRL secretion by altering the activity of inhibitory interneurons that synapse on the TIDA neurons. These effects are complicated by opposing intracellular signaling pathways linked to D1 and D2 receptors located on different populations of interneurons.272 The binding of dopamine or selective agonists such as bromocriptine to the D2 receptor has multiple effects on lactotroph function. D2 receptors are coupled to pertussis toxin–sensitive G proteins and inhibit adenylyl cyclase and decrease intracellular cAMP levels. Other effects include activation of an inwardly rectifying K+ channel, increase of voltage-activated K+ currents, decrease of voltage-activated Ca2+ currents, and inhibition of inositol phosphate production. Together, this spectrum of intracellular signaling events decreases free Ca2+ concentrations and inhibits exocytosis of PRL secretory granules.273 Dopamine also has a modest effect on thyrotrophs to inhibit the secretion of TSH. There is continuing debate concerning the mechanism by which D2 receptor activation inhibits transcription of the PRL gene. Likely pathways involve inhibition of MAPK or protein kinase C, with a resultant reduction in the phosphorylation of Ets family transcription factors. Ets factors are important for the stimulatory responses of TRH, insulin, and epidermal growth factor on PRL expression,274-276 and they interact cooperatively with the pituitary-specific protein PIT1 (a member of the POU domain family of transcription factors), which is essential for cAMP-mediated PRL gene expression.277 The second-messenger pathways used by the D2 receptor to inhibit lactotroph cell division are also unsettled. A study using primary pituitary cultures from rats demonstrated that forskolin treatment, which activates protein kinase A and elevates intracellular cAMP, or insulin treatment, which activates a potent receptor tyrosine kinase, were both effective mitogenic stimuli for lactotrophs. Bromocriptine competitively antagonized the proliferative response caused by elevated cAMP. Furthermore, inhibition of MAPK signaling by PD98059 markedly suppressed the mitogenic action of both insulin and forskolin, suggesting an interaction of MAPK and protein kinase A signaling.278 Another line of study has implicated the stimulation of phopholipase D activity by a Rho A–dependent, pertussis toxin–insensitive pathway in the antiproliferative effects of D2 receptor activation in both GH4C1 pituitary cells and NCI-H69 small cell lung cancer cells.279 Therefore, it is clear that dopamine actions on lactotrophs involve multiple different intracellular signaling pathways linked to activation of the D2 receptor, but different combinations of these pathways are relevant for the inhibitory effects on PRL secretion, PRL gene transcription, and lactotroph proliferation. The other major action of dopamine in the pituitary is inhibition of hormone secretion from the POMC-expressing

cells of the intermediate lobe—although, as noted earlier, the adult human differs from most other mammals in the rudimentary nature of this lobe. THDA and PHDA axon terminals provide a dense plexus of synaptic-like contacts on melanotrophs. Dopamine release from these terminals is inversely correlated with serum MSH levels280 and also regulates POMC gene expression and melanotroph proliferation.281 Other hypothalamic factors probably play a role secondary to that of dopamine as additional PIFs.263 The primary reason to conjecture the existence of these PIFs is the frequent inconsistency between portal dopamine levels and circulating PRL in different rat models. GABA is the strongest candidate and most likely acts through GABAA ionotropic receptors in the anterior pituitary. Melanotrophs, like lactotrophs, are inhibited by both dopamine and GABA but with the principal involvement of G protein– coupled, metabotropic GABAB receptors.282 Because basal dopamine tone is high, the measurable inhibitory effects of GABA on PRL release are generally small under normal circumstances. Other putative PIFs include somatostatin and calcitonin.

Prolactin-Releasing Factors Although tonic suppression of PRL release by dopamine is the dominant effect of the hypothalamus on PRL secretion, a number of stimuli promote PRL release, not merely by disinhibition of PIF effects but by causing release of one or more neurohormonal PRFs (see Fig. 7-27). The most important of the putative PRFs are TRH, oxytocin, and VIP, but AVP, angiotensin II, NPY, galanin, substance P, bombesinlike peptides, and neurotensin can also trigger PRL release under different physiologic circumstances.263 TRH has already been discussed. In humans, there is an imperfect correlation between pulsatile PRL and TSH release, suggesting that TRH cannot be the sole physiologic PRF under basal conditions.283 Like TRH, oxytocin, AVP, and VIP fulfill all the basic criteria for a PRF. They are produced in PVH neurons that project to the median eminence. Concentrations of the hormones in portal blood are much higher than in the peripheral circulation and are sufficient to stimulate PRL secretion in vitro. Moreover, there are functional receptors for each of the neurohormones in the anterior pituitary gland, and either pharmacologic antagonism or passive immunization against each hormone can decrease PRL secretion, at least under certain circumstances.284-288 AVP is released during stress and hypovolemic shock, as is PRL, suggesting a specific role for AVP as a PRF in these contexts. Similarly, another candidate PRF, peptide histidine isoleucine, may be specifically involved in the secretion of PRL in response to stress. Peptide histidine isoleucine and the human homologue PHM are structurally related to VIP and are synthesized from the same prohormone precursor in their respective species.289 Both peptides are coexpressed with CRH in parvicellular PVH neurons, and presumably they are released by the same stimuli that cause release of CRH into the hypophyseal-portal vessels.290 There is evidence suggesting that dopamine itself may also act as a PRF, in contrast to its predominant function as a PIF.263 At concentrations 3 orders of magnitude lower than that associated with maximal inhibition of PRL secretion, dopamine was shown to be capable of stimulating secretion from primary cultures of rat pituitary cells.291 These studies were extended to an in vivo model by Arey and colleagues,292 who demonstrated that low-dose dopamine infusion in cannulated rats caused a further increase in circulating PRL above the already elevated baseline

148    Neuroendocrinology produced by pharmacologic blockade of endogenous dopamine biosynthesis. The physiologic relevance of these findings to humans has yet to be established. Finally, reports of newly recognized PRFs continue to be published. Much excitement was generated by the isolation of a mammalian RFamide peptide from bovine hypothalamus named prolactin-releasing peptide (PrRP).293,294 PrRP binds with high affinity to its G protein–coupled receptor GPR10, expressed in human pituitary; it selectively stimulates PRL release from rat pituitary cells with a potency lower than that of TRH by itself, but synergistically in combination with TRH. However, PrRP is expressed predominantly in a subpopulation of noradrenergic neurons in the medulla and a small population of nonneurosecretory neurons of the VMH, raising the serious question about whether PrRP reaches the anterior pituitary and actually causes PRL secretion. Subsequent studies found no direct evidence for release of PrRP in the arcuate nucleus or median eminence, further suggesting that the peptide is not a hypophyseotropic neurohormone. PrRP probably does function as a neuromodulator within the CNS at sites expressing its receptor and probably is involved in the neural circuitry mediating stress responses and satiety.294,295

Intrapituitary Regulation of Prolactin Secretion Probably more than that of any other pituitary hormone, the secretion of PRL is regulated by autocrine-paracrine factors within the anterior lobe and by neurointermediate lobe factors that gain access to venous sinusoids of the anterior lobe by way of the short portal vessels. The wealth of local regulatory mechanisms within the anterior lobe has been reviewed extensively263,296 and is also discussed in Chapter 8. Galanin, VIP, endothelin-like peptides, angiotensin II, epidermal growth factor, basic fibroblast growth factor, GnRH, and the cytokine IL-6 are among the most potent local stimulators of PRL secretion. Locally produced inhibitors include PRL itself, acetyl­ choline, transforming growth factor-β, and calcitonin. Although none of these stimulatory or inhibitory factors plays a dominant role in the regulation of lactotroph function and much of the research in this area has not been directly confirmed in human pituitary, it seems apparent that the local milieu of autocrine and paracrine factors plays an essential modulatory role in determining the responsiveness of lactotrophs to hypothalamic factors in different physiologic states. As noted earlier, a proportion of the inhibitory dopamine tone affecting the anterior lobe lactotrophs is derived from the neurointermediate lobe. It was therefore unanticipated that surgical removal of this structure in rats would block suckling-induced PRL release over the moderate basal increase attributed to partial dopamine disinhibition.297 Further studies showed that exposure of the anterior pituitary to intermediate-lobe extracts (devoid of VIP, AVP, and other known PRFs) stimulated PRL secretion. At least two kinds of PRF activity have been isolated from intermediatelobe tumors of the mouse, but the specific molecules involved have yet to be identified.298 Other researchers have suggested a more passive role for the neurointermediate lobe in the regulation of PRL secretion. Melanotrophderived N-acetylated MSH appears to act as a lactotroph responsiveness factor by recruiting nonsecretory cells to an active state and sensitizing secreting lactotrophs to the actions of other direct PRFs.299 However, the relevance of the neurointermediate lobe for PRL regulation in primates (including humans) is not clear because of its attenuated structure in these species.

Neuroendocrine Regulation of Prolactin Secretion Secretion of PRL, like that of other anterior pituitary hormones, is regulated by hormonal feedback and neural influences from the hypothalamus.263,264,300 Feedback is exerted by PRL itself at the level of the hypothalamus. PRL secretion is regulated by many physiologic states including the estrous and menstrual cycles, pregnancy, and lactation. PRL is stimulated by several exteroceptive stimuli including light, ultrasonic vocalization of pups (in rodents), olfactory cues, and various modalities of stress. Expression and secretion of PRL are also influenced strongly by estrogens at the level of both the lactotrophs and the TIDA neurons301 (see Fig. 7-27) and by paracrine regulators within the pituitary such as galanin and VIP.

Feedback Control Negative feedback control of PRL secretion is mediated by a unique short-loop mechanism within the hypothalamus.302 PRL activates PRL receptors, which are expressed on all three subpopulations of A12 and A14 dopamine neurons, leading to increased tyrosine hydroxylase expression and increased dopamine synthesis and release.301,303 Ames dwarf mice that secrete virtually no PRL, GH, or TSH have decreased numbers of arcuate dopamine neurons, and this hypoplasia can be reversed by neonatal administration of PRL, suggesting a trophic action on the neurons.304 However, another mouse model of isolated PRL deficiency generated by gene targeting appears to have normal numbers of hypofunctioning dopamine neurons secondary to the loss of PRL feedback.305

Neural Control Lactotrophs have spontaneously high secretory activity, and therefore the predominant effect of the hypothalamus on PRL secretion is tonic suppression, which is mediated by regulatory hormones synthesized by tuberohypophyseal neurons. Secretory bursts of PRL are caused by the acute withdrawal of dopamine inhibition, stimulation by PRFs, or combinations of both events. At any given moment, locally produced autocrine and paracrine regulators further modulate the responsiveness of individual lactotrophs to neurohormonal PIFs and PRFs. Multiple neurotransmitter systems impinge on the hypothalamic dopamine and PRF neurons to regulate their neurosecretion (see Fig. 7-27).263 Nicotinic cholinergic and glutamatergic afferents activate TIDA neurons, whereas histamine, acting predominantly through H2 receptors, inhibits these neurons. An inhibitory peptidergic input to TIDA neurons of major physiologic significance is that associated with the endogenous opioid peptides enkephalin and dynorphin and their cognate µ- and κ-receptor subtypes.306 Opioid inhibition of dopamine release has been associated with increased PRL secretion under virtually all physiologic conditions, including the basal state, different phases of the estrous cycle, lactation, and stress. Ascending serotoninergic inputs from the dorsal raphe nucleus are the major activator of PRF neurons in the PVH.307 There is still debate concerning the identity of the specific 5-HT receptors involved in this activation. The PRL regulatory system and its monoaminergic control have been scrutinized in detail because of the frequent occurrence of syndromes of PRL hypersecretion (see Chapter 8). Both the pituitary and the hypothalamus have dopamine receptors, and the response to dopamine receptor stimulation and blockade does not distinguish between central and peripheral actions of the drug. Many commonly used neuroleptic drugs influence PRL secretion. Reserpine

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(a catecholamine depletor) and phenothiazines such as chlorpromazine and haloperidol enhance PRL release by disinhibition of dopamine action on the pituitary, and the PRL response is an excellent predictor of the antipsychotic effects of phenothiazines because of its correlation with D2 receptor binding and activation.308 The major antipsychotic neuroleptic agents act on brain dopamine receptors in the mesolimbic system and in the pituitary-regulating tuberoinfundibular system. Consequently, treatment of such patients with dopamine agonists such as bromocriptine can reverse the psychiatric benefits of these drugs. A report of three patients with psychosis and concomitant prolactinomas recommended the combination of clozapine and quinagolide as the treatment of choice to manage both diseases simultaneously.309

Factors Influencing Secretion Circadian Rhythm.  PRL is detectable in plasma at all times during the day but is secreted in discrete pulses superimposed on basal secretion, exhibiting a diurnal rhythm with peak values in the early morning hours.310 In humans, this is a true circadian rhythm, because it is maintained in a constant environment independently of the sleep rhythm.311 The combined body of data examining TIDA neuronal activity, dopamine concentrations in the median eminence, and manipulations of the SCN suggests that endogenous diurnal alterations in dopamine tone that are entrained by light constitute the major neuroendocrine mechanism underlying the circadian rhythm of PRL secretion. External Stimuli.  The suckling stimulus is the most important physiologic regulator of PRL secretion. PRL levels rise within 1 to 3 minutes after nipple stimulation, and they remain elevated for 10 to 20 minutes.312 This reflex is distinct from the milk let-down, which involves oxytocin release from the neurohypophysis and contraction of mammary alveolar myoepithelial cells. These reflexes provide a mechanism by which the infant regulates both the production and the delivery of milk. The nocturnal rise in PRL secretion in nursing and non-nursing women may have evolved as a mechanism of milk maintenance during prolonged nonsuckling periods at night. Pathways involved in the suckling reflex arise in the nerves innervating the nipple, enter the spinal cord by way of spinal afferent neurons, ascend the spinal cord through spinothalamic tracts to the midbrain, and enter the hypothalamus by way of the median forebrain bundle (see Fig. 7-27). Neurons regulating the oxytocin-dependent milk let-down response accompany those involved in PRL regulation throughout most of this pathway and then separate at the level of the PVH nuclei. The suckling reflex brings about an inhibition of PIF activity and a release of PRFs, although an undisputed suckling-induced PRF has not been identified. Although their significance for PRL regulation in humans is not certain, environmental stimuli arising from seasonal changes in light duration and auditory and olfactory cues are clearly of great importance to many mammalian species.263 Seasonal breeders, such as sheep, exhibit a reduction in PRL secretion in response to shortened days. The specific ultrasound vocalization of rodent pups is among the most potent stimuli for PRL secretion in lactating and virgin female rats. Olfactory stimuli from pheromones also have potent actions in rodents. A prime example is the Bruce effect, or spontaneous abortion induced by exposure of a pregnant female rat to an unfamiliar male. It is mediated by a well-studied neural circuitry involving the

vomeronasal nerves, the corticomedial amygdala, and the medial preoptic area of the hypothalamus, which results in activation of TIDA neurons and a reduction in circulating PRL that is essential for maintenance of luteal function in the first half of pregnancy. Stress in many forms dramatically affects PRL secretion, although the teleologic significance is uncertain. It may be related to actions of PRL on cells of the immune system or some other aspect of homeostasis. Different stressors are associated with either a reduction or an increase in PRL secretion, depending on the local regulatory environment at the time of the stress. However, whereas well-documented changes in PRL are associated with relatively severe forms of stress in laboratory animal models, a study of academic stress in college students failed to show any significant correlation of diurnal PRL levels with the time periods before, during, or after final examinations.313

Gonadotropin-Releasing Hormone and Control of the Reproductive Axis Chemistry and Evolution GnRH is the 10-amino-acid hypothalamic neuropeptide that controls the function of the reproductive axis. It is synthesized as part of a larger precursor molecule that is enzymatically cleaved to remove a signal peptide from the N-terminus and GnRH-associated peptide (GAP) from the C-terminus (Fig. 7-28).314 All forms of the decapeptide have a pyroGlu at the N-terminus and Gly-amide at the C-terminus, indicating the functional importance of the terminal residues throughout evolution. Two genes encoding GnRH have been identified within mammals.315,316 The first, GNRH1, encodes a 92-amino-acid precursor protein. This is the form of GnRH that is found in hypothalamic neurons and serves as a releasing factor to regulate pituitary gonadotroph function.317 The second GnRH gene, GNRH2, encodes a decapeptide that differs from the first by three amino acids.318 This form of GnRH is found in the midbrain region and serves as a neurotransmitter rather than as a pituitary releasing factor. Both GnRH-I and GnRH-II are found in phylogenetically diverse species, from fish to mammals, suggesting that these multiple forms of GnRH diverged from one another early in vertebrate evolution.317 A third form of GnRH, GnRH-III, has been identified in neurons of the telencephalon in teleost fish. GnRH is also found in cells outside the brain. The roles of GnRH peptides produced outside the brain are not well understood but are an area of current investigation. All GnRH genes have the same basic structure, with the pre-prohormone mRNA encoded in four exons. Exon 1 contains the 5′ untranslated region of the gene; exon 2 contains the signal peptide, GnRH, and the N-terminus of GAP; exon 3 contains the central portion of GAP; and exon 4 contains the C-terminus of GAP and the 3′ untranslated region (see Fig. 7-28).317 Among species, the nucleotide sequences encoding the GnRH decapeptide are highly homologous. This chapter focuses on the hypothalamic GnRH that is derived from GNRH1 mRNA and plays an important role in the regulation of the hypothalamicpituitary-gonadal axis. Two transcriptional start sites have been identified in the rat Gnrh1 gene, at the +1 and −579 positions, with the +1 promoter being active in hypothalamic neurons and the other promoter active in placenta. The first 173 base pairs of the promoter are highly conserved among species. In the rat, this promoter region has been shown to contain

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Figure 7-28  Schematic diagram of the human gene for gonadotropin-releasing hormone I (GNRH1), the hypothalamic complementary DNA (cDNA), and post-translational processing of the GnRH peptide. A cluster of binding sites for the homeodomain transcription factor BRN2 is present in both the proximal promoter and a distal enhancer region and is important for neuron-specific expression of the gene. Phylogenetically conserved homologous regions have been identified in the rat GnRH-I gene, but in that species the OCT1 transcription factor has been implicated in neuron-specific expression. The cDNA for GnRH-I isolated from human placenta has a longer 5′ untranslated region (UTR) because of differential splicing of the heterogeneous nuclear RNA (hnRNA) and inclusion of intron A sequences. GAP, GnRH-associated peptide; PAM, peptidylglycine α-amidating monooxygenase; TATA, Goldstein-Hogness box involved in binding RNA polymerase. (Compiled from data of Cheng CK, Leung PCK. Molecular biology of gonadotropin-releasing hormone (GnRH)-I, GnRH-II, and their receptors in humans. Endocr Rev. 2005;26:283-306; Wolfe A, Kim HH, Tobet S, et al. Identification of a discrete promoter region of the human GnRH gene that is sufficient for directing neuron-specific expression: a role for POU homeodomain transcription factors. Mol Endocrinol. 2002;16:435-449.)

two Oct1 binding sites; three regions that bind the POU domain family of transcription factors (Scip, Oct6, and Tst1); and three regions that can bind the progesterone receptor.319 In addition, a variety of hormones and second messengers have been shown to regulate GnRH gene expression, and the majority of the cis-acting elements that have been characterized for hormonal control of GnRH transcription are located in the proximal promoter region.320,321 The 5′ flanking region of the rodent and human GnRH-I genes also contain a distal 300-base-pair enhancer region that is 1.8 or 0.9 kb, respectively, upstream of the transcription start site.321,322 Studies have implicated the homeodomain transcription factors OCT1, MSX, and DLX in the specification of neuron expression and developmental activation.322,323

Anatomic Distribution GnRH neurons are small, diffusely located cells that are not concentrated in a discrete nucleus. They are generally bipolar and fusiform in shape, with slender axons projecting predominantly to the median eminence and infundibular stalk. The location of hypothalamic GnRH neurons is species dependent. In the rat, hypothalamic GnRH neurons are concentrated in rostral areas, including the medial preoptic area, the diagonal band of Broca, the septal areas, and the anterior hypothalamus. In humans and nonhuman primates, the majority of hypothalamic GnRH neurons are located more dorsally in the medial basal

hypothalamus, the infundibulum, and the periventricular region. Throughout the hypothalamus, neurohypophyseal GnRH neurons are interspersed with non-neuroendocrine GnRH neurons that extend their axons to other regions of the brain including other hypothalamic regions and various regions of the cortex. GnRH secreted from nonneuroendocrine neurons has been implicated in the control of sexual behavior in rodents but not in higher primates.324

Embryonic Development GnRH neuroendocrine neurons are an unusual neuronal population in that they originate outside the CNS, from the epithelial tissue of the nasal placode.325 During embryonic development, GnRH neurons migrate across the surface of the brain and into the hypothalamus, with the final hypothalamic location differing somewhat among species. Migration is dependent on a scaffolding of neurons and glial cells along which the GnRH neurons move, with neural cell adhesion molecules playing a critical role in guiding the migration process. In contrast to this widely accepted view of GnRH development, more recent data have suggested an alternative embryonic origin of GnRH neurons from the anterior pituitary placode and cranial neural crest.30 Failure of GnRH neurons to migrate properly leads to a clinical condition, Kallmann’s syndrome, in which GnRH neuroendocrine neurons do not reach their final

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destination and therefore do not stimulate pituitary gonadotropin secretion.326 Patients with Kallmann’s syndrome do not enter puberty spontaneously. The X-linked form of Kallmann’s syndrome results from a deficiency of the KAL1 gene, which encodes the extracellular glycoprotein termed anosmin-1. Loss-of-function mutations in the fibroblast growth factor receptor type 1 gene (FGFR1) produce an autosomal dominant form of Kallmann’s syndrome. This, together with other known genetic mutations in FGF8, prokinectin receptor 2 (PROKR2), and prokinectin 2 (PROK2), still account for only 30% of cases, and other lesions are yet to be characterized.327 Administration of exogenous GnRH effectively treats this form of hypothalamic hypogonadism. Patients with Kallmann’s syndrome often have other congenital midline defects, including anosmia, which results from hypoplasia of the olfactory bulb and tracts.

Action at the Pituitary Receptors.  GnRH binds to a membrane receptor on pituitary gonadotrophs and stimulates synthesis and secretion of both LH and FSH. The GnRH receptor is a seventransmembrane-domain G protein–coupled receptor, but it lacks a typical intracellular C-terminal cytoplasmic domain.321 Under physiologic conditions, GnRH receptor number varies and is usually directly correlated with the gonadotropin secretory capacity of pituitary gonadotrophs. For example, across the rat estrous cycle, a rise in GnRH receptors is seen just before the surge of gonadotropins that occurs on the afternoon of proestrus. GnRH receptor message levels are regulated by a variety of hormones and second messengers, including steroid hormones (estradiol can both suppress and stimulate; progesterone suppresses), gonadotropins (which suppress), and calcium and protein kinase C (which stimulate).321 Gq/11 is the primary guanosine triphosphate–binding protein mediating GnRH responses; however, there is evidence that GnRH receptors can couple to other G proteins, including Gs and Gi.321 With activation, the GnRH receptor couples to a phosphoinositide-specific phospholipase C, which leads to increases in calcium transport into gonadotrophs and calcium release from internal stores through a diacylglycerol-protein kinase C pathway. Increased calcium entry is a critical step in GnRH-stimulated release of gonadotropin secretion. However, GnRH also stimulates the MAPK cascade. When there is a decline in GnRH stimulation to the pituitary, as occurs in a variety of physiologic conditions including states of lactation, undernutrition, or seasonal periods of reproductive quiescence, the number of GnRH receptors on pituitary gonadotrophs declines dramatically. Subsequent exposure of the pituitary to pulses of GnRH restores receptor number by a Ca2+-dependent mechanism that requires protein synthesis.328 The effect of GnRH to induce its own receptor is termed upregulation or selfpriming. Only certain physiologic frequencies of pulsatile GnRH can augment GnRH receptor production, and these frequencies appear to differ among species.329 Upregulation of GnRH receptors after a period of low GnRH stimulation to the pituitary can take hours to days of exposure to pulsatile GnRH, depending on the duration and extent of the prior decrease in GnRH. The self-priming effect of GnRH to upregulate its own receptors also plays a crucial role in the production of the gonadotropin surge that occurs at midcycle in females of spontaneously ovulating species and triggers ovulation. Just before the gonadotropin surge, two factors—the increased frequency of pulsatile GnRH release and a sensitization of the pituitary gonadotrophs

by rising levels of estradiol—make the pituitary exquisitely sensitive to GnRH and allow an output of LH that is an order of magnitude greater than the release seen during the rest of the female reproductive cycle. This surge of LH triggers the ovulatory process at the ovary. In contrast to upregulation of GnRH receptors by pulsatile regimens of GnRH, continuous exposure to GnRH leads to downregulation of GnRH receptors and an accompanying decrease in LH and FSH synthesis and secretion, termed desensitization.330 Downregulation does not require calcium mobilization or gonadotropin secretion. It involves a rapid uncoupling of receptor from G proteins and sequestration of the receptors from the plasma membrane, followed by internalization and proteolytic degradation of the receptors. The concept of downregulation has a number of clinical applications. For example, the most common current therapy for precocious puberty of hypothalamic origin (i.e., precocious GnRH secretion) is to treat it with a longacting GnRH “superagonist” that downregulates pituitary GnRH receptors and effectively turns off the reproductive axis.329,331 Children with precocious puberty can be maintained with long-acting GnRH agonists for years to suppress the premature activation of the reproductive axis, and at the normal age of puberty agonist treatment can be withdrawn, allowing reactivation of pituitary gonadotrophs and a downstream increase in gonadal steroid hormone production (also see Chapter 25). Long-acting GnRH agonists are also used in the treatment of forms of breast cancer that are estrogen dependent and of other gonadal steroid-dependent cancers.329 Long-acting antagonists of GnRH have been developed that can also be used for these therapies.332 Antagonists have the advantage of not having a flare effect; that is, an acute stimulation of gonadotropin secretion that is seen during the initial treatment of individuals with superagonists. Pulsatile Gonadotropin-Releasing Hormone Stimulation.  Because a single pulse of GnRH stimulates the release of both LH and FSH and chronic exposure of the pituitary to pulsatile GnRH supports the synthesis of both LH and FSH, it is generally believed that there is only one releasing factor regulating the synthesis and secretion of LH and FSH. However, because there are divergent patterns of LH and FSH secretion in a number of physiologic conditions, a second FSH-releasing peptide has been proposed, but such a peptide has not been isolated to date. Other mechanisms, discussed in more detail later, are likely to account for the differential regulation of LH and FSH release. The ensemble of GnRH neurons in the hypothalamus that send axons to the portal blood system in the median eminence fire in a coordinated, repetitive, episodic manner, producing distinct pulses of GnRH in the portal bloodstream.333 The pulsatile nature of GnRH stimulation of the pituitary leads to the release of distinct pulses of LH into the peripheral circulation. In experimental animals, in which it is possible to collect blood samples simultaneously from the portal and peripheral blood, GnRH and LH pulses have been found to correspond in an approximately 1 : 1 ratio at most physiologic rates of secretion.334 Because the portal blood stream is generally inaccessible in humans, the collection of frequent peripheral venous blood samples is used to define the pulsatile nature of LH secretion (i.e., frequency and amplitude of LH pulses), and pulsatile LH is used as an indirect measure of the activity of the GnRH secretory system. Indirect assessment of GnRH secretion by monitoring the rate of pulsatile LH secretion is also used in many animal studies examining the factors that govern

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Regulatory Systems Many neurotransmitter systems from the brain stem, limbic system, and other areas of the hypothalamus convey information to GnRH neurons (Fig. 7-30). These afferent systems include neurons that contain norepinephrine, dopamine, serotonin, GABA, glutamate, endogenous opiate peptides, NPY, galanin, and a number of other peptide neurotransmitters. Glutamate and norepinephrine play important roles in providing stimulatory drive to the

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recovery from chronic undernutrition, the ratio of FSH to LH is higher than when it is measured in adults experiencing regular reproductive function. As discussed later, steroid hormones act at both the hypothalamus and the pituitary to influence strongly the rate of pulsatile GnRH release and the amount of LH and FSH secreted from the pituitary. GnRH pulse frequency not only influences the rate of pulsatile gonadotropin release and the ratio of FSH to LH secretion but also plays an important role in modulating the structural makeup of the gonadotropins. LH and FSH are structurally similar glycoprotein hormones. Each of these hormones is made up of an α and a β subunit. LH, FSH, and TSH share a common α-subunit, and each has a unique β subunit that conveys receptor specificity to the intact hormone. Before secretion of gonadotropins, terminal sugars are attached to each gonadotropin molecule.116 The sugars include sialic acid, galactose, N-acetylglucosamine, and mannose, but the most important is sialic acid. The extent of glycosylation of LH and FSH is important for the physiologic function of these hormones.116 Forms of gonadotropin with more sialic acid have a longer half-life because they are protected from degradation by the liver. Forms of gonadotropin with less sialic acid can have more potent effects at their biologic receptors. Both the rate of GnRH stimulation and ovarian hormone feedback at the level of the pituitary regulate the degree of LH and FSH glycosylation. For example, slow frequencies of GnRH release, seen during follicular development, are associated with greater degrees of FSH glycosylation, which would provide sustained FSH support to growing follicles. In contrast, faster frequencies of GnRH release, seen just before the midcycle gonadotropin surge, are associated with lesser degrees of FSH glycosylation, providing a more potent but shorter-lasting form of FSH at the time of ovulation.337

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the regulation of the pulsatile activity of the reproductive neuroendocrine axis. Unlike LH secretion, FSH secretion is not always pulsatile, and even when it is pulsatile, there is only partial concordance between LH and FSH pulses. It is possible to place multiple-unit recording electrodes in the medial basal hypothalamus of monkeys and other species and find spikes of electrical activity that are concordant with the pulsatile secretion of LH secretion.335 However, it is unknown whether these bursts of electrical activity reflect the activity of GnRH neurons themselves or the activity of neurons that impinge on GnRH neurons and govern their firing. With the development of mice in which the gene for green fluorescent protein has been put under the regulation of the GnRH promoter, it has been possible to identify GnRH neurons in hypothalamic tissue slices using fluorescence microscopy and to record from them intracellularly.14 These studies have shown that many, but not all, GnRH neurons show a bursting pattern of electrical activity. A central, unsolved question in the field of reproductive neuroendocrinology is what causes GnRH neurons to pulse in a coordinated manner. Studies using a line of clonal GnRH neurons have shown that these neurons grown in culture can release GnRH in a pulsatile pattern, suggesting that the pulse-generating capacity of GnRH neurons is intrinsic.336 The term GnRH pulse generator is often used to acknowledge the fact that GnRH secretion occurs in pulses and to refer to the central mechanisms responsible for pulsatile GnRH release. A critical factor governing LH and FSH secretion is the rate of pulsatile GnRH stimulation of the gonadotrophs. Experimental studies in which the hypothalamus was lesioned and GnRH was replaced by pulsatile administration of exogenous GnRH showed that different frequencies of GnRH can lead to different ratios of LH to FSH secretion from the pituitary. Figure 7-29 shows that in a monkey with a hypothalamic lesion, replacement of one pulse of GnRH per hour led to a relatively low ratio of FSH to LH secretion. Subsequent institution of a slower pulse frequency (one pulse of GnRH every 3 hours) led to a decrease in LH secretion but an increase in FSH secretion, so that the ratio of FSH to LH secretion was greatly elevated. It is likely that this effect of pulse frequency on the ratio of FSH to LH secretion accounts, at least in part, for the clinical finding that at times when the GnRH pulse generator is just turning on, such as at the onset of puberty and during

Figure 7-29 The influence of gonadotropinreleasing hormone (GnRH) pulse frequency on luteinizing hormone (LH) and follicle-stimulating hormone (FSH) secretion in a female rhesus monkey with an arcuate nucleus lesion ablating endogenous GnRH support of the pituitary. Decreasing the GnRH pulse frequency from 1 pulse every hour to 1 pulse every 3 hours led to a decrease in plasma LH concentrations but an increase in plasma FSH concentrations. (Redrawn from Wildt L, Haulser A, Marshall G, et al. Frequency and amplitude of gonadotropin-releasing hormone stimulation and gonadotropin secretion in the rhesus monkey. Endocrinology. 1981;109:376-385.)

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Figure 7-30 Regulation of the hypothalamic-pituitary-gonadal axis. Schematic diagram of the hypothalamic-pituitary-gonadal axis showing neural systems that regulate gonadotropin-releasing hormone (GnRH) secretion and feedback of gonadal steroid hormones at the level of the hypothalamus and pituitary. CNS, central nervous system; CRH, corticotropin-releasing hormone; FSH, follicle-stimulating hormone; GABA, γ-aminobutyric acid; GALP, galanin-like peptide; LH, luteinizing hormone; NE, norepinephrine; NPY, neuropeptide Y.

reproductive axis, whereas GABA and endogenous opioid peptides provide a substantial portion of the inhibitory drive to GnRH neurons. Influences of specific neurotransmitter systems are discussed where appropriate in the following sections, which cover the physiologic regulation of GnRH neurons. GnRH neurons are surrounded by glial processes, and only a small percentage of their surface area is available to receive dendritic contacts from afferent neurons. Changes in the steroid hormone milieu influence the degree of glial sheathing and may play important roles in regulating afferent input to GnRH neurons by this mechanism.43,45 Some glial cells also secrete substances including transforming growth factor-α and prostaglandin E2 that can modulate the activity of GnRH neurons.

Feedback Regulation Steroid hormone receptors are abundant in the hypothalamus and in many neural systems that impinge on GnRH neurons, including noradrenergic, serotoninergic, βendorphin–containing, and NPY neurons. Early studies identifying regions of the brain that bind labeled estrogens showed that in rodents the preoptic area and the VMH had the highest concentrations of estrogen receptors in the brain. Further localization studies, identifying estrogen receptors by immunocytochemistry or in situ hybridization, confirmed the strong presence of estrogen receptors in the hypothalamus and in brain areas with abundant connections to the hypothalamus, including the amygdala, septal nuclei, bed nucleus of the stria terminalis,

154    Neuroendocrinology medial part of the nucleus of the solitary tract, and lateral portion of the parabrachial nucleus.338 In 1986, a new member of the steroid hormone receptor superfamily with high sequence homology to the classic estrogen receptor (now referred to as estrogen receptor-α) was isolated from rat prostate and named estrogen receptor-β. This novel estrogen receptor was shown to bind estradiol and to activate transcription by binding to estrogen response elements.339 In situ hybridization studies examining the localization of estrogen receptor-β mRNA have shown that these receptors are present throughout the rostral-caudal extent of the brain, with a high level of expression in the preoptic area, bed nucleus of the stria terminalis, PVH and SON nuclei, amygdala, and laminae II to VI of the cerebral cortex.340 Specific receptors for progesterone are induced by estrogen in hypothalamic regions of the brain, including the preoptic area, the ventromedial and ventrolateral nuclei, and the infundibular-arcuate nucleus, although there is also evidence for constitutive expression of progesterone receptors in some regions.341 Androgen receptor mapping studies have shown considerable overlap in the distribution of androgen and estrogen receptors throughout the brain. The highest density of androgen receptors was found in hypothalamic nuclei known to participate in the control of reproduction and sexual behaviors, including the arcuate nucleus, PVH, medial preoptic nucleus, ventromedial nucleus, and brain regions with strong connections to the hypothalamus including the amygdala, nuclei of the septal region, bed nucleus of the stria terminalis, nucleus of the solitary tract, and lateral division of the parabrachial nucleus.338 The anterior pituitary also contains receptors for all of the gonadal steroid hormones. Steroid hormones can dramatically alter the pattern of pulsatile release of GnRH and of the gonadotropins through actions at both the hypothalamus and the pituitary. At the hypothalamus, estradiol, progesterone, and testosterone can all act to slow the frequency of GnRH release into the portal bloodstream as part of a closed negative feedback loop.342 Because GnRH neurons have generally been shown to lack steroid hormone receptors, it is likely that the effects of steroid hormones on the firing rate of GnRH neurons are mediated by steroid hormone actions on other neural systems that provide afferent input to GnRH neurons. For example, progesterone-mediated negative feedback on GnRH secretion in primates appears to be regulated by β-endorphin–containing neurons in the hypothalamus, acting primarily through µ-opioid receptors. If a µ-receptor antagonist, such as naloxone, is administered along with progesterone, the negative feedback action of progesterone on GnRH secretion can be blocked. Negative feedback of steroid hormones can also occur directly at the level of the pituitary. For example, estradiol has been shown to be capable of binding to the pituitary, decreasing LH and FSH synthesis and release, and decreasing the sensitivity of pituitary gonadotrophs to the actions of GnRH so that less LH and FSH are released when a pulse of GnRH stimulates the pituitary. Evidence for such a direct pituitary action of estradiol came from studies with rhesus monkeys that had been rendered deficient in endogenous GnRH by a lesion in the arcuate nucleus and showed a decline in endogenous gonadotropin secretion. When these monkeys received a pulsatile regimen of GnRH gonadotropin secretion, subsequent estradiol infusions dramatically suppressed the responsiveness of the pituitary to GnRH and suppressed the gonadotropin secretion that was being driven by the pulsatile administration of GnRH.343 Similarly, in a compound mutant mouse model on a GnRH-deficient (hpg) genetic background, expression

of a human FSH-β transgene was inhibited by testosterone directly at the pituitary level.344 In primate species including humans, there is considerable feedback of estradiol at the pituitary, but most of the progesterone- and testosterone-negative feedback occurs at the level of the hypothalamus.342 Most of the time, the hypothalamic-pituitary axis is under the negative feedback influence of gonadal steroid hormones. If the gonads are removed surgically or their normal secretion of steroid hormones is suppressed pharmacologically, there is a dramatic increase (10-fold to 20-fold) in circulating levels of LH and FSH secretion.342 This type of “castration response” occurs normally at menopause in women, when ovarian follicular development and, therefore, ovarian production of large quantities of estradiol and progesterone decrease and eventually cease. In addition to negative feedback, estradiol can have a positive feedback action at the level of the hypothalamus and pituitary leading to a massive release of LH and FSH from the pituitary. This massive release of gonadotropins occurs once each menstrual cycle and is referred to as the LH-FSH surge. The positive feedback action of estradiol occurs as a response to the rising tide of estradiol that is produced during the process of dominant follicle development in the late follicular phase of the menstrual cycle. In women, elevated estradiol levels are typically maintained at about 300 to 500 pg/mL for approximately 36 hours before the stimulation of the gonadotropin surge. Experiments have shown that both a critical concentration and a critical duration of elevated estradiol are necessary to achieve positive feedback and a resulting gonadotropin surge. If supraphysiologic doses of estradiol are administered, the surge can occur as early as 18 hours after their administration. Because the ovary is responsible for the production of estradiol and the time course and magnitude of estradiol release control the rate of positive feedback, the ovary has been referred to as the zeitgeber of the menstrual cycle. The dependence of the positive feedback system on the magnitude of estradiol production helps explain the fact that the portion of the menstrual cycle that varies most in length is the follicular phase. Production of higher levels of estradiol by a dominant follicle in one cycle leads to a more rapid positive feedback action, with earlier ovulation and therefore a shorter follicular phase, compared with a cycle in which the dominant follicle produced lower levels of estradiol. As with negative feedback in response to estradiol, the positive feedback actions of estradiol occur both at the hypothalamus, to increase GnRH secretion, and at the pituitary, to greatly enhance pituitary responsiveness to GnRH. Estradiol increases pituitary sensitivity to GnRH by increasing the synthesis of new GnRH receptors and by enhancing the responsiveness to GnRH at a postreceptor site of action. At the level of the hypothalamus in rodent species, estradiol appears to act at a “surge center” to induce the ovulatory surge of GnRH. Lesions in areas adjacent to the medial preoptic area, near the anterior commissure and septal complex, block the ability of estradiol to induce a surge in these species without blocking negative feedback effects of estradiol.345 In primate species, there does not appear to be a separate surge center mediating the positive feedback actions of estradiol. Cellular mechanisms that mediate the switch from negative to positive feedback of estrogen are not fully understood, but there is support for the concept that estrogen induction of various transcription factors and receptors (notably progesterone receptors) may play an important

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role in mediating this switch.346 The recent isolation of a novel mammalian RFamide peptide, named kisspeptin or metastin, that is the natural ligand for the former orphan G protein–coupled receptor GPR54 has shed considerable light on this area.294 Loss-of-function mutations in GPR54 (now termed KISSR) cause HH. Kisspeptin is expressed in subpopulations of arcuate and anteroventral periventricular (AVPV) neurons that project to GnRH neurons. Kisspeptin expression is regulated by estradiol and testosterone and is upregulated at the time of puberty, and intracerebroventricular administration of kisspeptin causes the secretion of GnRH and gonadotropins.347,348 Furthermore, kisspeptin expression in the AVPV, but not in the arcuate nucleus, is sexually diergic, with a much greater number of kisspeptin neurons in the female. This particular subpopulation of kisspeptin neurons is activated in an estrogen-dependent manner immediately preceding the GnRH surge, as detected by Fos expression, and is postulated to play a key role in the positive feedback effects of estradiol on GnRH release.349

have spontaneous ovarian cycles. In the human menstrual cycle, day 1 is designated as the first day of menstrual bleeding. At this time, small and medium-sized follicles are present in the ovaries and only small amounts of estradiol are being produced by the follicular cells. As a result, there is a low level of negative feedback to the hypothalamic-pituitary axis, LH pulse frequency is relatively fast (one pulse about every 60 minutes), and FSH concentrations are slightly elevated compared with much of the rest of the cycle (Fig. 7-31). FSH acts at the level of the ovarian follicles to stimulate development and causes an increase in follicular estradiol production, which in turn provides increased negative feedback to the hypothalamic-pituitary unit. A result of the increased negative feedback is a slowing of pulsatile LH secretion over the course of the follicular phase, to a rate of about one pulse every 90 minutes. However, as the growing follicle (or follicles, depending on the species) secretes more estradiol, a positive feedback action of estradiol is triggered and leads to an increase in GnRH release and a surge release of LH and FSH. The surge of gonadotropins acts at the fully developed follicle to stimulate dissolution of the follicular wall, leading to ovulation of the matured ovum into the nearby fallopian tube, where fertilization takes place if sperm are present. Ovulation results in a reorganization of the cells of the follicular wall, which undergo hypertrophy and hyperplasia and start to secrete large amounts of progesterone and some

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Figure 7-31 Diagrammatic representation of changes in plasma levels of estradiol, progesterone, luteinizing hormone (LH), and follicle-stimulating hormone (FSH) and in portal levels of gonadotropin-releasing hormone (GnRH) over the human menstrual cycle.

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GnRH (pg/mL) (Hypophyseal Portal)

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156    Neuroendocrinology estradiol. Progesterone and estradiol have a negative feedback effect at the level of the hypothalamus and pituitary, so the LH pulse frequency becomes very slow during the luteal phase of the menstrual cycle. The corpus luteum has a fixed life span, and without additional stimulation in the form of chorionic gonadotropin from a developing embryo, the corpus luteum regresses spontaneously after about 14 days and secretion of progesterone and estradiol diminishes. This reduces the negative feedback signals to the hypothalamus and pituitary and allows an increase in FSH and LH secretion. The fall in progesterone is also a withdrawal of steroid hormone support to the endometrial lining of the uterus; as a result, the endometrium is shed as menses, and a new cycle begins. In other species, the interplay between the neuroendocrine and ovarian hormones is similar but the timing of events is different and other factors, such as circadian and seasonal regulatory factors, play a role in regulating the cycle. The rat has a 4- or 5-day ovarian cycle with no menses (the endometrial lining is absorbed rather than shed). The rat also shows strong circadian rhythmicity in the timing of the LH-FSH surge, with the surge always occurring in the afternoon of the day of proestrus. The sheep is an example of a species that has a strongly seasonal pattern of ovarian cyclicity. During the breeding season, sheep have 15-day cycles, with a very short follicular phase and an extended luteal phase; during the nonbreeding season, signals relaying information about day length through the visual system, pineal, and SCN cause a dramatic suppression of GnRH neuronal activity, and cyclic ovarian function is prevented by a decrease in trophic hormonal support from the pituitary.

Early Development and Puberty Neuroendocrine stimulation of the reproductive axis is initiated during fetal development, and in primates in mid­ gestation circulating levels of LH and FSH reach values similar to those in castrated adults.350 Later in gestational development, gonadotropin levels decline, restrained by rising levels of circulating gonadal steroids. The steroids that have this effect are probably placental in origin, because after parturition there is a rise in circulating gonadotropin levels that is apparent for a variable period in the first year of life, depending on the species.351 The decline in reproductive hormone secretion in the postnatal period appears to be caused by a decrease in GnRH stimulation of the reproductive axis because it occurs even in the castrate state and because gonadotropin and gonadal steroid secretion can be supported by administration of pulses of GnRH.352 Pubertal reawakening of the reproductive axis occurs in late childhood and is marked initially by nighttime elevations in gonadotropin and gonadal steroid hormone levels.352,353 The mechanisms controlling the pubertal reawakening of the GnRH pulse generator have been an area of intense investigation for more than 2 decades.352 Although the mechanisms are not fully understood, significant progress has been made in identifying central changes in the hypothalamus that appear to play a role in this process. At puberty, there is both a decrease in transsynaptic inhibition to the GnRH neuronal system and an increase in stimulatory input to GnRH neurons.352 One of the major inhibitory inputs to the GnRH system is provided by GABAergic neurons. Studies in rhesus monkeys have shown that hypothalamic levels of GABA decrease during early puberty and that blocking the GABAergic input before puberty, by intrahypothalamic administration of antisense oligodeoxynucleotides against the enzymes responsible for

GABA synthesis, results in premature activation of the GnRH neuronal system. It has been suggested, on the basis of findings that a subset of glutamate receptors (i.e., kainate receptors) increase in the hypothalamus at puberty, that the pubertal decrease in GABA tone may be caused by an increase in glutamatergic transmission. Further evidence pointing to a role for glutamate comes from studies showing that administration of N-methyl-D-aspartic acid (NMDA) to prepubertal rhesus monkeys can drive the reawakening of the reproductive axis.354 Increased stimulatory drive to the GnRH neuronal system also appears to come from increases in norepinephrine and NPY at the time of puberty.352 Furthermore, as discussed earlier, there is evidence that growth factors act through release of prostaglandin from glial cells at puberty to play a role in stimulating GnRH neurons.355 Despite an increased understanding of the neural changes occurring at puberty, the question of what signals trigger the pubertal awakening of the reproductive axis is unanswered at this time.356 However, the kisspeptin neuron system (described earlier) has become a prime integrative candidate for this function.349 Also relevant to pubertal onset was the discovery of galanin-like peptide (GALP), which is expressed specifically in the arcuate nucleus and binds with high affinity to galanin receptors. GALP is a potent central stimulator of gonadotropin release and sexual behavior in the rat and can reverse the decreased reproductive function associated with diabetes mellitus and hypoinsulinemia.357 Both kisspeptin and GALP neurons are targets of leptin and are hypothesized to be involved in the well-known modulation of puberty and reproductive function by food availability and nutritional status (described in the next section).

Reproductive Function and Stress Many forms of physical stresses, such as energy restriction, exercise, temperature stress, infection, pain, and injury, as well as psychological stresses, such as being subordinate in a dominance hierarchy or being acutely psychologically stressed, can suppress the activity of the reproductive axis.358 If the stress exposure is brief, there may be acute suppression of circulating gonadotropins and gonadal steroid hormones; in females, normal menstrual cyclicity may be disrupted, but fertility is unlikely to be impaired.358 In contrast, prolonged periods of significant stress exposure can lead to complete impairment of reproductive function, also characterized by low circulating levels of gonadotropins and gonadal steroids. Stress appears to decrease the activity of the reproductive axis by decreasing GnRH drive to the pituitary, because in all cases in which it has been examined, administration of exogenous GnRH can reverse the effects of the stress-induced decline in reproductive hormone secretion. Although the neural circuits through which many forms of stress suppress GnRH neuronal activity are not known, some forms of stress-induced suppression of reproductive function are better understood. In the case of foot-shock stress in rats359 or immune stress (i.e., injection of IL-1α) in primates,360 the suppression of gonadotropin secretion that occurs was shown to be reversible by administration of a CRH antagonist, implying that endogenous CRH secretion mediates the effects of these stresses on GnRH neurons. In other studies, naloxone, a µ-opioid receptor antagonist, was shown to be capable of reversing restraint stress–induced suppression of gonadotropin secretion in monkeys; however, naloxone was ineffective in reversing the suppression of gonadotropin secretion that occurs during insulin-induced hypoglycemia.361,362 In the case of metabolic stresses, multiple

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regulators appear to mediate changes in the neural drive to the reproductive axis. Various metabolic fuels including glucose and fatty acids can regulate the function of the reproductive axis, and blocking cellular utilization of these fuels can lead to suppression of gonadotropin secretion and decreased gonadal activity. Leptin, a hormone produced by fat cells, can also modulate the activity of the reproductive axis. Mutant ob/ob mice that are deficient in leptin or leptin receptors are infertile, and fertility can be restored by administration of leptin.363 Moreover, leptin administration has been shown to reverse the suppressive effects of undernutrition on the reproductive axis in some situations.364 Leptin receptors are found in several cell populations that are known to have a strong influence on the reproductive axis, particularly NPY and kisspeptin neurons.349 In summary, it appears that a number of neural circuits can mediate effects of stress on the GnRH neuronal system

and that the neural systems involved are at least somewhat specific to the type of stress that is experienced.

NEUROENDOCRINE DISEASE Disease of the hypothalamus can cause pituitary dysfunction, neuropsychiatric and behavioral disorders, and disturbances of autonomic and metabolic regulation. In the diagnosis and treatment of suspected hypothalamic or pituitary disease, four issues must be considered: the extent of the lesion, the physiologic impact, the specific cause, and the psychosocial setting. The etiology of hypothalamic neuroendocrine disorders categorized by age and syndrome is summarized in Tables 7-7 and 7-8. Manifestations of pituitary insufficiency secondary to hypothalamic or pituitary stalk damage are not identical to those of primary pituitary insufficiency. Hypothalamic injury causes decreased secretion of most pituitary

TABLE 7-7 

Etiology of Hypothalamic Disease by Age Premature Infants and Neonates

10 to 25 yr

Intraventricular hemorrhage Meningitis: bacterial Tumors: glioma, hemangioma Trauma Hydrocephalus, kernicterus

Tumors Craniopharyngioma Glioma, hamartoma, dysgerminoma Histiocytosis X, leukemia Dermoid, lipoma, neuroblastoma Trauma Vascular Subarachnoid hemorrhage Aneurysm Arteriovenous malformation Inflammatory disease Meningitis Encephalitis Sarcoidosis Tuberculosis Structural brain defect Chronic hydrocephalus Increased intracranial pressure

1 mo to 2 yr Tumors Glioma, especially optic glioma Histiocytosis X Hemangioma Hydrocephalus Meningitis Familial disorders Laurence-Moon-Biedl syndrome Prader-Willi syndrome 2 to 10 yr Neoplasms Craniopharyngioma Glioma, dysgerminoma, hamartoma Histiocytosis X, leukemia Ganglioneuroma, ependymoma Medulloblastoma Meningitis Bacterial meningitis Tuberculous meningitis Encephalitis Viral Exanthematous demyelinating Familial Diabetes insipidus Radiation therapy Diabetic ketoacidosis Moyamoya disease, circle of Willis

25 to 50 yr Nutritional: Wernicke’s disease Tumors Glioma, lymphoma, meningioma Craniopharyngioma, pituitary tumors Angioma, plasmacytoma, colloid cysts Ependymoma, sarcoma, histiocytosis X Inflammatory disease Sarcoidosis Tuberculosis, viral encephalitis Vascular Aneurysm, subarachnoid hemorrhage Arteriovenous malformation Damage from pituitary radiation therapy 50 yr and Older Nutritional: Wernicke’s disease Tumors: pituitary tumors, sarcoma, glioblastoma, ependymoma, meningioma, colloid cysts, lymphoma Vascular disease Infarct, subarachnoid hemorrhage Pituitary apoplexy Inflammatory disease: encephalitis, sarcoidosis, meningitis Damage from radiation therapy for ear-nose-throat carcinoma, pituitary tumors

Adapted from Plum F, Van Uitert R. Nonendocrine diseases and disorders of the hypothalamus. In: Reichlin S, Baldessarini RJ, Martin JB, eds. The Hypothalamus, vol 56. New York, NY: Raven Press, 1978:415-473.

158    Neuroendocrinology TABLE 7-8 

Etiology of Endocrine Syndromes of Hypothalamic Origin Hypophyseotropic Hormone Deficiency Surgical pituitary stalk section Inflammatory disease: basilar meningitis and granuloma, sarcoidosis, tuberculosis, sphenoid osteomyelitis, eosinophilic granuloma Craniopharyngioma Hypothalamic tumor: infundibuloma; teratoma (ectopic pinealoma); neuroglial tumor, particularly astrocytoma Maternal deprivation syndrome, psychosocial dwarfism Isolated GHRH deficiency Hypothalamic hypothyroidism Panhypophyseotropic failure Disorders of Regulation of GnRH Secretion Female Precocious puberty: GnRH-secreting hamartoma, hCG-secreting germinoma Delayed puberty Neurogenic amenorrhea Pseudocyesis Anorexia nervosa “Functional” amenorrhea and oligomenorrhea Drug-induced amenorrhea Male Precocious puberty Fröhlich’s syndrome Olfactory-genital dysplasia (Kallmann’s syndrome) Disorders of Regulation of Prolactin-Regulating Factors Tumor Sarcoidosis Drug-induced Reflex Herpes zoster of chest wall Post-thoracotomy Nipple manipulation Spinal cord tumor “Psychogenic” Hypothyroidism Carbon dioxide narcosis Disorders of Regulation of CRH Paroxysmal corticotropin discharge (Wolff ’s syndrome) Loss of circadian variation Depression CRH-secreting gangliocytoma CRH, corticotropin-releasing hormone; hCG, human chorionic gonadotropin; GHRH, growth hormone–releasing hormone; GnRH, gonadotropin-releasing hormone.

hormones but can cause hypersecretion of hormones that are normally under inhibitory control by the hypothalamus, as in hypersecretion of PRL after damage to the pituitary stalk and precocious puberty caused by loss of the normal restraint over gonadotropin maturation. Impairment of inhibitory control of the neurohypophysis can lead to the syndrome of inappropriate vasopressin secretion (SIADH) (see Chapter 10). More subtle abnormalities in secretion can result from impairment of the control system. For example, loss of the normal circadian rhythm of ACTH secretion may occur before loss of pituitary-adrenal secretory reserve, and responses to physiologic stimuli may be paradoxical. Because hypophyseotropic hormone levels cannot be measured directly and pituitary hormone secretion is regulated by complex, multilayered controls, assay of pituitary hormones in blood does not necessarily give a

meaningful picture of events at hypothalamic and higher levels. Rarely, tumors secrete excessive amounts of releasing peptides and cause hypersecretion of hormones from the pituitary. Disorders of the hypothalamic-pituitary unit can result from lesions at several levels. Defects can arise from destruction of the pituitary (as by tumor, infarct, inflammation, or autoimmune disease) or from a hereditary deficiency of a particular hormone (as in rare cases of isolated FSH, GH, or POMC deficiency). Selective loss of thyroid hormone receptors in the pituitary can give rise to increased TSH secretion and thyrotoxicosis. Furthermore, disorders can arise through disruption of the contact zone of the stalk and median eminence, the stalk itself, or the nerve terminals of the tuberohypophyseal system; such disruption occurs after surgical stalk section, with tumors involving the stalk, and in some inflammatory diseases. At a higher level, tonic inhibitory and excitatory inputs can be lost; this is manifested by absence of circadian rhythms or by the development of precocious puberty. Physical stress, cytokine products of inflammatory cells, toxins, and reflex inputs from peripheral homeostatic monitors also impinge on the tuberoinfundibular system. At the highest level of control, emotional stress and psychological disorders can activate the pituitary-adrenal stress response and suppress gonadotropin secretion (e.g., psychogenic amenorrhea) or inhibit GH secretion (e.g., psychosocial dwarfism) (see Chapter 17). Intrinsic disease of the anterior pituitary is reviewed in Chapter 8, and disturbances in neurohypophyseal function are discussed in Chapter 10. This chapter considers primarily diseases of the hypothalamic-pituitary unit.

Pituitary Isolation Syndrome Destructive lesions of the pituitary stalk, such as may occur with head injury, surgical transection, tumor, or granuloma, produce a characteristic pattern of pituitary dysfunction.365-367 Central diabetes insipidus (DI) develops in a large percentage of patients, depending on the level at which the stalk has been sectioned. If the cut is close to the hypothalamus, DI is almost always produced, but if the section is low on the stalk, the incidence is lower. The extent to which nerve terminals in the upper stalk are preserved determines the clinical course. The classic triphasic syndrome of initial polyuria followed by normal water control and then by AVP deficiency over a period of 1 week to 10 days occurs in fewer than half of the patients. The sequence is attributed to an initial loss of neurogenic control of the neural lobe, followed by autolysis of the neural lobe with release of AVP into the circulation and finally by complete loss of AVP. However, full expression of polyuria requires adequate cortisol levels; if cortisol is deficient, AVP deficiency may be present with only minimal polyuria. DI can also develop after stalk injury without an overt transitional phase. When DI occurs after head injury or operative trauma, varying degrees of recovery can be seen even after months or years. Sprouting of nerve terminals in the stump of the pituitary stalk may give rise to sufficient functioning tissue to maintain water balance. In contrast to the effects of stalk section, nondestructive injury to the neurohypophysis or stalk, such as during surgical resection of optic chiasmatic astrocytomas, can sometimes give rise to transient SIADH.368 Although head injury, granulomas, and tumors are the most common causes of acquired DI, other cases develop in the absence of a clearcut cause.369 Autoimmune disease of the hypothalamus may be the cause in some instances, as was suggested by the finding of autoantibodies to

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neurohypophyseal cells in one third of patients with “idiopathic” DI in one series.370 However, autoantibodies were also frequently found in association with histiocytosis X. Later reports suggested the importance of continued vigilance in cases of idiopathic DI. A definite cause is frequently uncovered in time, including a high proportion of occult germinomas, whose detection by MRI may be preceded by elevated levels of human chorionic gonadotropin (hCG) in CSF.371,372 Congenital DI can be part of a hereditary disease. DI in the Brattleboro rat is caused by an autosomal recessive genetic defect that impairs production of AVP but not of oxytocin. In contrast, inherited forms of DI in humans have been attributed to mutations in the vasopressin V2 receptor gene or, less frequently, in the aquaporin or the AVP gene.373-376 Menstrual cycles cease after stalk section, although gonadotropins may still be detectable, unlike the situation after hypophysectomy. Plasma glucocorticoid levels and urinary excretion of cortisol and 17-hydroxycorticoids decline after hypophysectomy and stalk section, but the change is slower after stalk section. A transient increase in cortisol secretion after stalk section is believed to result from the release of ACTH from preformed stores. The ACTH response to the lowering of blood cortisol is markedly reduced, but ACTH release after stress may be normal, possibly because of CRH-independent mechanisms. Reduction in thyroid function after stalk section is similar to that seen with hypophysectomy. The fall in GH secretion is said to be the most sensitive indication of damage to the stalk; however, the insidious nature of this endocrinologic change in adults who have suffered traumatic brain injury may cause it to be overlooked and therefore contribute to delayed rehabilitation.377 Humans with stalk sections or tumors of the stalk region have widely varying levels of hyperprolactinemia and may have galactorrhea.378 PRL responses to hypoglycemia and to TRH are blunted, in part because of loss of neural connections with the hypothalamus. PRL responses to dopamine agonists and antagonists in patients with pituitary isolation syndrome are similar to those in patients with prolactinomas. Interestingly, PRL secretion continues to show a diurnal variation in patients with either hypothalamopituitary disconnection or microprolactinoma.310 Both forms of hyperprolactinemia are characterized by a similarly increased frequency of PRL pulses and a marked rise in nonpulsatile or basal PRL secretion, although the disruption is greater in the tumoral hyperprolactinemia. An incomplete pituitary isolation syndrome may occur with the empty sella syndrome, intrasellar cysts, or pituitary adenomas.379-381 Anterior pituitary failure after stalk section is in part due to loss of specific neural and vascular links to the hypothalamus and in part to pituitary infarction.

Hypophyseotropic Hormone Deficiency Selective pituitary failure can be caused by a deficiency of specific pituitary cell types or a deficiency of one or more hypothalamic hormones. Isolated GnRH deficiency is the most common hypophyseotropic hormone deficiency. In Kallmann’s syndrome (gonadotropin deficiency commonly associated with hyposmia),326 hereditary agenesis of the olfactory lobe may be demonstrable by MRI.382 Abnormal development of the GnRH system is a result of defective migration of the GnRH-containing neurons from the olfactory nasal epithelium in early embryologic life (see earlier discussion). Other malformations of the cranial midline structures, such as absence of the septum pellucidum in

septo-optic dysplasia (De Morsier’s syndrome), can cause HH or, less commonly, precocious puberty. A surprisingly large percentage of children with septo-optic dysplasia who otherwise have multiple hypothalamic-pituitary abnormalities actually retain normal gonadotropin function and enter puberty spontaneously.383 The genetic basis of HH, including Kallmann’s syndrome, has now been established in approximately 15% of patients.327,384 Mutations in KAL1, the Kallmann’s syndrome gene, and in NROB1 (formerly AHC or DAX1), the gene that causes adrenal hypoplasia congenita with hypogonadotropic hypogonadism, produce X-linked recessive disease. Autosomal recessive HH has been associated with mutations in the genes encoding the GnRH receptor, KISS1 (metastin) receptor, leptin, leptin receptor, FSH, LH, PROP1 (combined pituitary deficiency), and HESX1 (septo-optic dysplasia), and deficient FGFR1 function causes an autosomal dominant form of HH. Mutations in PROK2 and PROKR2, which encode prokineticin 2 and its receptor, have been associated with heterozygous, homozygous, compound heterozygous, and oligogenic patterns of genetic penetrance. The GnRH response test is of little value in the differential diagnosis of HH. Most patients with GnRH deficiency show little or no response to an initial test dose but normal responses after repeated injection. This slow response has been attributed to downregulation of GnRH receptors in response to prolonged GnRH deficiency. In patients with intrinsic pituitary disease, the response to GnRH may be absent or normal. Consequently, it is not possible to distinguish between hypothalamic and pituitary disease with a single injection of GnRH. Prolonged infusions or repeated administration of GnRH agonists after hormone replacement therapy priming may aid in the diagnosis or provide therapeutic options for women with Kallmann’s syndrome who wish to become pregnant.385,386 Deficiency of TRH secretion gives rise to hypothalamic hypothyroidism, also called tertiary hypothyroidism, which can occur in hypothalamic disease or, more rarely, as an isolated defect.387 Molecular genetic analyses have revealed infrequent autosomal recessive mutations in the TRH and TRH receptor genes in the etiology of central hypothyroidism.388 Hypothalamic and pituitary causes of TSH deficiency are most readily distinguished by imaging methods. Although it is theoretically reasonable to use the TRH stimulation test for the differentiation of hypothalamic disease from pituitary disease, it is of limited value. The typical pituitary response to TRH administration in patients with TRH deficiency is an enhanced and somewhat delayed peak, whereas the response with pituitary failure is sub­ normal or absent. The hypothalamic type of response has been attributed to an associated GH deficiency that sensitizes the pituitary to TRH (possibly through suppression of somatostatin secretion), but GH also affects T4 metabolism and may alter pituitary responses as well.389 In practice, the responses to TRH in hypothalamic and pituitary disease overlap so much that this test cannot be used reliably for a differential diagnosis. Persistent failure to demonstrate responses to TRH is good evidence for the presence of intrinsic pituitary disease, but the presence of a response does not mean that the pituitary is normal. Deficient TRH secretion leads to altered TSH biosynthesis by the pituitary, including impaired glycosylation. Poorly glycosylated TSH has low biologic activity, and dissociation of bioactive and immunoreactive TSH can lead to the paradox of normal or elevated levels of TSH in hypothalamic hypothyroidism.387,390 GHRH deficiency appears to be the principal cause of hGH deficiency in children with idiopathic dwarfism.391

160    Neuroendocrinology

Craniopharyngioma Craniopharyngioma is the most common pediatric tumor occurring in the sellar and parasellar area (see Table 7-7). Because of their location, these benign neoplasms frequently cause significant neuroendocrine dysfunction. The more common adamantinomatous tumors in children

Short Stature Normal GH

50

GH (µg/L)

40 Hexarelin GHRH

30 20 10 0

GH Deficiency Intact Stalk

10

GH (µg/L)

8 Hexarelin GHRH

6 4 2 0

GH Deficiency Disrupted Stalk

10 8 GH (µg/L)

This condition is frequently associated with abnormal electroencephalograms, a history of birth trauma, and breech delivery, although a cause-and-effect relationship has not been established. MRI scans show that most children with isolated, idiopathic hGH deficiency have a normal sized or only slightly reduced anterior pituitary; less common findings are ectopic posterior pituitary, anterior pituitary hypoplasia, or empty sella.392 In contrast, children with idiopathic combined pituitary hormone deficiency are significantly more likely to have evidence of moderate to severe anterior pituitary hypoplasia, ectopic posterior pituitary, complete agenesis of the pituitary stalk (both nervous and vascular components), and a variety of associated midline cerebral malformations.392 Human GH is the most vulnerable of the anterior pituitary hormones when the pituitary stalk is damaged. It can be difficult to differentiate between primary pituitary disease and GHRH deficiency by standard tests of GH reserve. However, a substantial GH secretory response to a single administration of hexarelin occurs only in the presence of at least a partially intact vascular stalk (Fig. 7-32).217 In many children with dwarfism, the anatomic abnormalities of the intrasellar contents and pituitary stalk, together with the frequent occurrence of other midline defects, such as those observed in septo-optic dysplasia, are consistent with the alternative hypothesis of a developmental defect occurring in embryogenesis.392 There has been a remarkable advance in understanding of the molecular ontogeny of the hypothalamic-pituitary unit, much of it based on mutant mouse models.27 Parallel genetic analyses have been conducted in children with isolated GH deficiency or combined pituitary hormone deficiencies. These studies have identified autosomal recessive mutations in structural and regulatory genes including the genes encoding the GHRH receptor, PIT1, PROP1, HESX1, LHX3, and LHX4 that are responsible for a sizable proportion of congenital hypothalamic-pituitary disorders once considered idiopathic.206,207,391,392 Adrenal insufficiency is another manifestation of hypothalamic disease and rarely is caused by CRH deficiency.393 Isolated ACTH deficiency is uncommon, but there is suggestive evidence in at least one family of genetic linkage to the CRH gene locus.394 More recent investigations have revealed mutations in TBX19, the gene encoding TPIT, a T-box transcription factor expressed only in pituitary corticotrophs and melanotrophs, which is associated with the majority of cases of isolated ACTH deficiency in neonates.395 The CRH stimulation test does not reliably distinguish hypothalamic from pituitary failure as a cause of ACTH deficiency.396 Apart from intrinsic diseases of the hypothalamus such as tumors and granulomas, two environmental causes of central hypophyseotropic deficiencies are of increasing clinical importance: trauma to the brain,366,367,377 particularly from motor vehicle accidents, and the sequelae of chemotherapy and radiation therapy for intracranial lesions in children and adults.390,397,398 Improved short-term survival from head injuries associated with coma and CNS malignancies has greatly increased the prevalence of longterm neuroendocrine consequences.

Hexarelin GHRH

6 4 2 0 -15

0

15

30

45

60

75

90 105 120

Time (min) Figure 7-32 Effect of hypothalamic-pituitary disconnection on the growth hormone (GH) secretory responses to growth hormone–releasing hormone (GHRH) (1 µg/kg) and hexarelin (2 µg/kg) administered intravenously to children with GH deficiency. Top, Mean responses in a group of 24 prepubertal children with short stature secondary to familial short stature or constitutional growth delay are shown. Middle, Children with GH deficiency and an intact vascular pituitary stalk as visualized by dynamic magnetic resonance imaging exhibited a clear, but blunted, GH response to both secretagogues. Bottom, In contrast, children with pituitary stalk agenesis (both vascular and neural components) had no response or a markedly attenuated response to both peptides. (From Maghnie M, Spica-Russotto V, Cappa M, et al. The growth hormone response to hexarelin in patients with different hypothalamic-pituitary abnormalities. J Clin Endocrinol Metab. 1998;83: 3886-3889.)

usually contain both a cystic component filled with a turbid, cholesterol-rich fluid and a solid component characterized by organized epithelial cells.399 Roughly 25% of craniopharyngiomas are diagnosed in patients after the age of 25 years, and this subset of tumors is more typically papillary in nature, solid, and less likely to be calcified or cystic.399 Both forms of craniopharyngioma probably result from metaplastic changes in vestigial epithelial cell rests

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that originate in Rathke’s pouch and the craniopharyngeal duct during fetal development. Common presenting symptoms are those resulting from a mass intracranial lesion and increased intracranial pressure. Visual field defects, papilledema, and optic atrophy can occur from compression of the optic chiasm or nerves. Between 80% and 90% of affected children have signs and symptoms of endocrine dysfunction, although these are not usually the chief complaint. The most frequent hormone deficiencies are GH and gonadotropins. The latter is almost universal in adolescents and adults and probably is also present, but undetected, in prepubertal children with craniopharyngioma. TSH and ACTH deficiencies are also common. Even if not present at initial diagnosis, endocrine dysfunction often occurs subsequent to treatment and necessitates long-term follow-up and retesting.400 MRI is the imaging modality of choice in cases of suspected craniopharyngioma.401 A recommended examination includes T1-weighted thin sagittal and coronal sections, obtained before and after contrast administration, through the sella and suprasellar regions. T2-weighted and fluid attenuation inversion recovery (FLAIR) images are useful to further delineate cysts and are hyperintense. Computed tomography scans can be useful to determine the presence of calcification.

Hypophyseotropic Hormone Hypersecretion Pituitary hypersecretion is occasionally caused by tumors of the hypothalamus. GnRH-secreting hamartomas can cause precocious puberty.402 CRH-secreting gangliocytomas can cause Cushing’s syndrome,403 and GHRH-secreting gangliocytomas of the hypothalamus can cause acromegaly.404 Although they do not arise from the hypothalamus, paraneoplastic syndromes can also cause pituitary hypersecretion, as with CRH-secreting tumors and GHRHsecreting tumors of the bronchi and pancreas. Bronchial carcinoids and pituitary islet cell tumors are the usual causes of this phenomenon.

Neuroendocrine Disorders of Gonadotropin Regulation Precocious Puberty The term precocious puberty is used when physiologically normal pituitary-gonadal function appears at an early age (see also Chapter 25).405 By convention, it is defined as the onset of androgen secretion and spermatogenesis before the age of 9 or 10 years in boys or the onset of estrogen secretion and cyclic ovarian activity before age 7 or 8 in girls.406,407 Central precocious puberty is caused by disturbed CNS function, which may or may not have an identifiable structural basis. The term pseudoprecocious puberty refers to premature sexual development resulting from excessive secretion of androgens, estrogens, or hCG; it is caused by tumors (both gonadal and extragonadal), administration of exogenous gonadal steroids, or genetically determined activation of gonadotropin receptors (see Chapter 25). Central precocious puberty with neurogenic causes and pineal gland disease are discussed in this chapter. Idiopathic Sexual Precocity.  Familial occurrence of idiopathic sexual precocity is uncommon, but there is a hereditary form that is largely confined to boys. Abnormal

electroencephalograms and behavioral disturbances, suggesting the presence of brain damage, have been reported occasionally in girls with idiopathic precocious puberty. The pathogenesis may be related to the rate of hypothalamic development or other, undetermined nutritional, environmental, or psychosocial factors. Many cases previously thought to be idiopathic are caused by small hypothalamic hamartomas (see later discussion). It has been argued that localized activation of discrete cellular subsets connected to GnRH neurons may be sufficient to initiate puberty.408 Neurogenic Precocious Puberty.  Approximately two thirds of hypothalamic lesions that influence the timing of human puberty are located in the posterior hypothalamus, but in the subset of patients who come to autopsy, damage is extensive. Specific lesions known to cause precocity include craniopharyngioma (although delayed puberty is far more common with these lesions), astrocytoma, pineal tumors, subarachnoid cysts, encephalitis, miliary tuberculosis, tuberous sclerosis or neurofibromatosis type 1, the Sturge-Weber syndrome, porencephaly, craniostenosis, microcephaly, hydrocephalus, empty sella syndrome, and Tay-Sachs disease.409,410 Hamartoma of the hypothalamus is an exception to the generalization that tumors of the brain cause precocious puberty by impairment of gonadotropin secretion (although hamartomas on occasion cause hypothalamic damage). A hamartoma is a tumor-like collection of normal-appearing nerve tissue lodged in an abnormal location. The parahypothalamic type consists of an encapsulated nodule of nerve tissue attached to the floor of the third ventricle or suspended from the floor by a peduncle; it is typically less than 1  cm in diameter. The intrahypothalamic or sessile type is enveloped by the posterior hypothalamus and can distort the third ventricle. These hamartomas tend to be larger than the pedunculated variety; they grow in the interpeduncular cistern and are frequently accompanied by seizures, mental retardation, and developmental delays. They result in precocious puberty with about half the incidence observed with the parahypothalamic lesions.411,412 Before the development of high-resolution scanning techniques, this tumor was considered rare, but small ones can now be visualized. Miniature hamartomas of the tuber cinereum are commonly discovered at autopsy. Precocious puberty occurs when the hamartoma makes connections with the median eminence and thus serves as an accessory hypothalamus. Peptidergic nerve terminals containing GnRH have been found in these tumors.402 Early pubertal development is presumably caused by unrestrained GnRH secretion, although the hamartomas almost certainly have an intrinsic pulse generator of GnRH secretion, because pulsatility is required for stimulation of gonadotropin secretion (see earlier discussion). Manifestations of premature puberty in patients with hamartomas are similar to those associated with other central causes of precocity. Hamartomas occur in both sexes and may be present as early as age 3 months. In the past, most cases were thought to be fatal by age 20 years, but many hamartomas cause no brain damage and need not be excised.411 The interpeduncular fossa of the brain is difficult to approach, and surgical experience is somewhat limited. Early in the course of illness, epilepsy manifested as “brief, repetitive, stereotyped attacks of laughter”413 may provide a clue to the disease. Late in the course, hypothalamic damage can cause severe neurologic defects and intractable seizures.

162    Neuroendocrinology Hypothyroidism.  Hypothyroidism can cause precocious puberty in girls that is reversible with thyroid therapy. Hyperprolactinemia and galactorrhea may be present. One possibility is that elevated TSH levels (in children with thyroid failure) cross-react with the FSH receptor.414 Alternatively, low levels of thyroid hormone might simultaneously activate release of LH, FSH, and TSH. A third possibility is that hypothyroidism causes hypothalamic encephalopathy that impairs the normal tonic suppression of gonadotropin release by the hypothalamus. The high PRL levels that sometimes accompany this disorder may result from a deficiency in PIF secretion, increased secretion of TRH, or increased sensitivity of the lactotrophs to TRH secretion. Tumors of the Pineal Gland.  Pineal gland tumors account for only a small percentage of intracranial neoplasms. They occur as a central midline mass with an enhancing lesion on MRI frequently accompanied by hydrocephalus. Pinealomas cause a variety of neurologic abnormalities. Parinaud’s syndrome, which consists of paralysis of upward gaze, pupillary areflexia (to light), paralysis of convergence, and a wide-based gait, occurs with about half of patients with pinealomas. Gait disturbances can also occur because of brain stem or cerebellar compression. Additional neurologic signs occurring with moderate frequency include spasticity, ataxia, nystagmus, syncope, vertigo, cranial nerve palsies other than VI and VIII, intention tremor, scotoma, and tinnitus. Several discrete cytopathologic entities account for mass lesions in the pineal region (Table 7-9).415 The most common non-neoplastic conditions are degenerative pineal cysts, arachnoid cysts, and cavernous hemangioma. Pinealocytes give rise to primitive neuroectodermal tumors, the so-called small blue-cell tumors that are immunopositive for the neuronal marker synaptophysin and negative for the lymphocyte marker CD45. True pinealomas can be relatively well differentiated pineocytomas, intermediate mixed forms, or the less differentiated pineoblastomas,415,416 the latter of which are essentially identical to medulloblastomas, neuroblastomas, and oat cell carcinomas of the lung. The most common tumors of the pineal gland are actually germinomas (a form of teratoma), so designated because of their presumed origin in germ cells. Germinomas may also occur in the anterior hypothalamus or the floor of the third ventricle, where they are often associated with the clinical triad of DI, pituitary insufficiency, and visual abnormalities.410 Identical tumors can be found in the testis and anterior mediastinum. Intracranial germinomas have a tendency to spread locally, infiltrate the hypothalamus, and metastasize to the spinal cord and CSF. Extracranial metastases (to skin, lung, or liver) are rare. Teratomas derived from two or more germ cell layers also occur in the pineal region. Chorionic tissue in teratomas and germinomas may secrete hCG in sufficient amounts to cause gonadal maturation, and some of these tumors have histologic and functional characteristics of choriocarcinomas. Diagnosis is confirmed by the combination of a mass lesion, cytologic analysis of CSF, and radioimmunoassay detection of hCG in the CSF. Precocious puberty is a relatively unusual manifestation of pineal gland disease. When it occurs, neuroanatomic studies suggest that the cause is secondary to pressure or destructive effects of the pineal tumor on the function of the adjacent hypothalamus, or to the secretion of hCG. Most patients have other evidence of hypothalamic involvement, such as DI, polyphagia, somnolence, obesity, or behavioral disturbance. Choriocarcinoma of the pineal gland is associated with high plasma levels of hCG. The

TABLE 7-9 

Classification of Tumors of the Pineal Region A. Germ Cell Tumors 1.  Germinoma a. Posterior third ventricle and pineal lesions b. Anterior third ventricle, suprasellar, or intrasellar lesions c. Combined lesions in anterior and posterior third ventricle, apparently noncontiguous, with or without foci of cystic or solid teratoma 2. Teratoma a. Evidencing growth along two or three germ lines in varying degrees of differentiation b. Dermoid and epidermoid cysts with or without solid foci of teratoma c. Histologically malignant forms with or without differentiated foci of benign, solid, or cystic teratoma-teratocarcinoma, chorioepithelioma, embryonal carcinoma (endodermal–sinus tumor or yolk-sac carcinoma); combinations of these with or without foci of germinoma, chemodectoma B. Pineal Parenchymal Tumors 1. Pinealocytes a. Pineocytoma b. Pineoblastoma c. Ganglioglioma and chemodectoma d. Mixed forms exhibiting transitions between these 2. Glia a. Astrocytoma b. Ependymoma c. Mixed forms and other less frequent gliomas (e.g., glioblastoma, oligodendroglioma) C. Tumors of Supporting or Adjacent Structures 1. Meningioma 2. Hemangiopericytoma D. Nonneoplastic Conditions of Neurosurgical Importance 1. “Degenerative” cysts of pineal gland lined by fibrillary astrocytes 2. Arachnoid cysts 3. Cavernous hemangioma From DeGirolami U. Pathology of tumors of the pineal region. In: Schmidek HH, ed. Pineal Tumors. New York, NY: Masson; 1977:1-19.

hCG can stimulate testosterone secretion from the testis but not estrogen secretion by the ovary; it therefore causes premature puberty almost exclusively in boys. The prevalence of elevated hCG levels in children with premature puberty related to tumors in the pineal region is unknown, but the fact that this phenomenon occurs further challenges the theory that nonparenchymal tumors cause precocious puberty by damaging the normal pineal gland. Rarely, pinealomas cause delayed puberty, raising speculation about a role of melatonin in inhibiting gonadotropin secretion in these cases. Management of tumors in the pineal region is not straightforward.415,417 Operative mortality rates can be high, but the rationale for an aggressive approach is based on the need to make a histologic diagnosis, the variety of lesions found in this region, the possibility of cure of an encapsulated lesion, and the effectiveness of chemotherapeutic agents for germinomas and choriocarcinoma. Stereotaxic biopsy of the pineal region provided diagnosis in 33 of 34 cases in one series, suggesting that this is a useful alternative to open surgical exploration for diagnostic purposes.418 Long-term palliation or cure of many pineal region tumors is possible through combinations of surgery, radiation, gamma knife, or chemotherapy, depending on the nature of the lesion.419 Approach to the Patient with Precocious Puberty.  Several groups have reviewed the diagnostic approach to suspected

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central precocious puberty (see Chapter 25).420,421 Although guidelines differ, the index of suspicion is clearly inversely proportional to the age of the patient. A GnRH stimulation test to assess gonadotropin release and thereby differentiate between primed and inactive gonadotrophs is probably the single most important endocrinologic measure. If LH and FSH levels are not stimulated and there is no evidence of gonadal germ cell maturation, the cause of precocious puberty lies outside the hypothalamic-pituitary axis, and the diagnostic process should focus on the adrenal glands and gonads (see Chapters 15 and 25). MRI studies are central to the workup for exclusion or characterization of organic lesions in the areas of the sella, optic chiasm, suprasellar hypothalamus, and interpeduncular cistern.422 Management of Sexual Precocity.  Structural lesions of the hypothalamus are treated by surgery, radiation, chemotherapy, or combinations of these treatments, as indicated by the pathologic diagnosis and extent of disease. Endocrinologic manifestations of precocious puberty are best treated by GnRH agonists with the therapeutic goals of delaying sexual maturation to a more appropriate age and achieving optimal linear growth and bone mass, possibly with the combined use of GH treatment.423,424 Other approaches include the use of cyproterone acetate, testolactone, or spironolactone to antagonize or inhibit gonadal steroid biosynthesis.425,426 Precocious puberty is stressful to both the child and the parents, and it is essential that psychological support be provided.

Psychogenic Amenorrhea Menstrual cycles can cease in young nonpregnant women with no demonstrable abnormalities of the brain, pituitary, or ovary in several situations,427,428 including pseudocyesis (false pregnancy), anorexia nervosa, excessive exercise, psychogenic disorders, and hyperprolactinemic states (see Chapter 17). Psychogenic amenorrhea, the most common cause of secondary amenorrhea except for pregnancy, can occur with major psychopathology or minor psychic stress and is often temporary. Psychogenic amenorrhea is probably mediated by excessive endogenous opioid activity, because naloxone or naltrexone (both opiate receptor blockers) can induce ovulation in some patients with this disorder.428 Exercise-induced amenorrhea may be a variant of psychogenic amenorrhea, or it may result from loss of body fat.427,429 The syndrome is associated with intense and prolonged physical exertion such as running, swimming, or ballet dancing. Affected women are always at less than ideal body weight and have low stores of fat. If the activity is begun before puberty, normal sexual maturation can be delayed for many years. Fat mass may be a regulator of gonadotropin secretion, with adipocyte-derived leptin as the principal mediator between peripheral energy stores and hypothalamic regulatory centers.430 Studies in nonhuman primates showed a direct role of caloric intake in the pathogenesis of amenorrhea associated with long-distance running.431 Exercise and psychogenic amenorrhea can have adverse effects because of the associated estrogen deficiency and accompanying osteopenia (also see Chapter 28).432

Neurogenic Hypogonadism in Males A discussion of neurogenic hypogonadism in males should begin with an account of Fröhlich’s syndrome (adiposogenital dystrophy), which was originally characterized as delayed puberty, hypogonadism, and obesity associated with a tumor that impinges on the hypothalamus.1 It was

later recognized that either hypothalamic or pituitary dysfunction can induce hypogonadism, and the presence of obesity indicates that the appetite-regulating regions of the hypothalamus have been damaged. Several organic lesions of the hypothalamus can cause this syndrome, including tumors, encephalitis, microcephaly, Friedreich’s ataxia, and demyelinating diseases. Other important causes of HH are Kallmann’s syndrome, a disorder caused by failure of GnRH-containing neurons to migrate normally (see earlier discussions of GnRH and hypophyseotropic hormone deficiency), and a subset of the Prader-Willi syndrome.433 However, most males with delayed sexual development do not have serious neurologic conditions. Furthermore, most obese boys with delayed sexual development have no structural damage to the hypothalamus but have constitutional delayed puberty, which is commonly associated with obesity. It is not known whether there is a functional disorder of the hypothalamus in this condition. It is thought that psychosexual development of brain maturation depends on the presence of androgens within a critical developmental window corresponding to puberty; therefore, hypogonadism in boys (regardless of cause) should be treated by the middle teen years (15 years of age at the latest). In adult men, hypogonadism (including reduced spermatogenesis) can be induced by emotional stress or severe exercise,434 but this abnormality is seldom diagnosed because the symptoms are more subtle than menstrual cycle changes in similarly stressed women. Prolonged physical stress and sleep and energy deficiencies can also decrease testosterone and gonadotropin levels.435 Chronic intrathecal administration of opiates for the control of intractable pain syndromes is strongly associated with HH, and to a lesser extent with hypocorticism and GH deficiency, in both men and women.436 Finally, critical illness with multiple causes is well known to be associated with hypogonadism and ineffectual altered pulsation of GnRH.437

Neurogenic Disorders of Prolactin Regulation Neurogenic causes of hyperprolactinemia include irritative lesions of the chest wall (e.g., herpes zoster, thoracotomy), excessive tactile stimulation of the nipple, and lesions within the spinal cord (e.g., ependymoma).438 Prolonged mechanical stimulation of the nipples by suckling or the use of a breast pump can initiate lactation in some women who are not pregnant, and neurologic lesions that interrupt the hypothalamic-pituitary connection can cause hyperprolactinemia, as discussed earlier. Hyperprolactinemia also occurs after certain forms of epileptic seizures. In one series, six of eight patients with temporal lobe seizures had a marked increase in PRL, whereas only one of eight patients with frontal lobe seizures led to hyperprolactinemia.439 Agents that block D2-like dopamine receptors (e.g., phenothiazines, later-generation atypical antipsychotics) or prevent dopamine release (e.g., reserpine, methyldopa) must be excluded in all cases. Because the nervous system exerts such profound effects on PRL secretion, patients with hyperprolactinemia (including those with adenomas) may have a deficit of PIF or an excess of PRF activity. In studies of PRL secretion in patients apparently cured of hyperprolactinemia by removal of a pituitary microadenoma, regulatory abnormalities persisted in some but not all patients. Regulatory abnormalities may persist because of incomplete removal of tumor, abnormal function of the remaining part of the gland, or underlying hypothalamic abnormalities.440

164    Neuroendocrinology

Neurogenic Disorders of Growth Hormone Secretion

neuroregulatory deficiency syndrome is therefore unclear, and the decision to treat short children with hGH should be made cautiously.448,449

Hypothalamic Growth Failure

Neurogenic Hypersecretion of Growth Hormone

Loss of the normal nocturnal increase in GH secretion and loss of GH secretory responses to provocative stimuli occur early in the course of hypothalamic disease and may be the most sensitive endocrine indicator of hypothalamic dysfunction. As described earlier, anatomic malformations of midline cerebral structures are associated with abnormal GH secretion, presumably related to failure of the development of normal GH regulatory mechanisms. Such disorders include optic nerve dysplasia and midline prosencephalic malformations (absence of the septum pellucidum, abnormal third ventricle, and abnormal lamina terminalis). Certain complex genetic disorders including Prader-Willi syndrome also commonly involve reduced GH secretory capacity.441 Idiopathic hypopituitarism with GH deficiency was considered earlier.

Diencephalic Syndrome.  Children and infants with tumors in and around the third ventricle frequently become cachectic, which is often associated with elevated hGH levels and paradoxical GH secretory responses to glucose and insulin.450 GH hypersecretion may be caused by a hypothalamic abnormality or by malnutrition. Deficits of pituitary-adrenal regulation are less common. A striking feature is an alert appearance and seeming euphoria despite the profound emaciation. A variety of associated neurologic abnormalities may be present, including nystagmus, irritability, hydrocephalus, optic atrophy, tremor, and excessive sweating. CSF abnormalities include increased protein and the presence of abnormal cells. Most cases are caused by chiasmatic-hypothalamic gliomas, with the majority classified as astrocytomas.450 Treatment options include surgical resection, radiation therapy, and chemotherapy.451

Maternal Deprivation Syndrome and Psychosocial Dwarfism Infant neglect or abuse can impair growth and cause failure to thrive (the maternal deprivation syndrome). Malnutrition interacts with psychological factors to cause growth failure in these children, and each case should be carefully evaluated from this point of view. Older children with growth failure in a setting of abuse or severe emotional disturbance (termed psychosocial dwarfism) may also have abnormal circadian rhythms and deficient hGH release after insulin-induced hypoglycemia or arginine infusion (see Chapter 24).442 Deficient release of ACTH and gonadotropins may also be present. A new variant, termed hyperphagic short stature, has been identified.443 These disorders can be reversed by placing the child in a supportive milieu; growth and neuroendocrine hGH responses rapidly return to normal.444 The pathogenesis of altered GH secretion in children in response to deprivation is unknown. Furthermore, in the adult human, physical or emotional stress usually causes an increase in hGH secretion (see earlier discussion).

Neuroregulatory Growth Hormone Deficiency The availability of biosynthetic hGH for treatment of short stature has brought into focus a group of patients who grow at low rates (